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CION Cancer Clinics
Testing healthy brothers and sisters for a family gene fault | CION Cancer Clinics
Healthy brothers and sisters of a child with an inherited cancer fault can be tested for that exact fault, but only when knowing early actually helps them. For syndromes where checks in childhood find tumours early, testing is usually offered promptly. For faults that raise risk only in adult life, it normally waits until the sibling can decide for themselves. This page explains how families make that call. At CION Cancer Clinics, our oncologists plan screening and care for families with an inherited cancer syndrome, explained in plain words.
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Medically reviewed by Dr. Naresh GunduConsultant Medical Oncologist · MBBS, DNB (Internal Medicine), DM (Medical Oncology, AIIMS) · last reviewed September 2026, next review due September 2027
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On this page
- Should a healthy brother or sister be tested?
- When is testing a sibling worth doing, and when should it wait?
- How is a sibling tested?
- What do the words the counsellor uses mean?
- What does a sibling's result change?
- What this page cannot tell you
- Four things families tell us, and what is actually true
- Common questions about testing siblings
The short answer
Should a healthy brother or sister be tested?
Yes, when knowing early would change how that child is looked after. For syndromes where regular checks in childhood find tumours sooner, siblings are usually offered testing promptly. For faults that raise risk only in adult life, testing normally waits until the sibling is old enough to choose for themselves.
The test is simple; the decision is not
Once the family fault is known, a sibling's test is a small blood or saliva sample checked for that one fault. The harder part is what comes with the answer. A positive result means years of checks for a child who feels perfectly well. A negative one can bring its own mixed feelings.
Why the parents' results come first
If one parent carries the fault, each sibling has an even, one in two chance of carrying it too. If neither parent does, the fault probably began in the affected child, and siblings have a much lower chance. Knowing this first helps the family decide how urgent sibling testing is.
Testing a sibling is about what the result lets you do, not about satisfying curiosity.Test now or wait
When is testing a sibling worth doing, and when should it wait?
The rule counsellors follow is simple. Test a child when the result leads to something that helps them in childhood.
Test in childhood
Some syndromes need checks that start in the early years, so a result now is genuinely useful.
Usually tested early
- Hereditary retinoblastoma, where eye checks start from birth
- Li-Fraumeni syndrome, linked to the TP53 gene
- Some thyroid and adrenal syndromes, such as MEN2
- Von Hippel-Lindau and bowel polyp syndromes
Wait until adulthood
Faults such as BRCA1 and BRCA2 raise risk in adult life, and nothing is done differently for a child who carries one. Testing waits so the young person can decide for themselves.
A newborn or a pregnancy
For some syndromes a baby can be tested soon after birth, sometimes from the umbilical cord blood. A negative result can spare a newborn repeated eye examinations under anaesthetic.
A sibling with a view
An older child or teenager should be part of the conversation. Their questions, fears and wishes count, and counsellors will ask for their agreement, not just their parents' consent.
Not sure whether this applies to you?
Ask an oncologistStep by step
How is a sibling tested?
A family counselling session
Parents meet the counsellor to talk through what a positive and a negative result would mean for this child. Older siblings join for part of the session.
Explaining it to the child
Children cope better when they know why they are giving blood. The counsellor can suggest words that suit the child's age, in Telugu or English, without frightening them.
A targeted sample
A small blood sample, or saliva for a younger child. The laboratory checks only the known family fault, so no full panel is needed.
Results and a plan
A positive result leads to a surveillance plan. A negative one usually means the child returns to ordinary care, with no extra checks for this fault.
In plain language
What do the words the counsellor uses mean?
- Predictive test
- A test on someone who is well, to see whether they carry a fault already found in the family.
- Known familial variant
- The exact fault found in the first person tested. Relatives are checked for that one spot only.
- Assent
- A child's own agreement, given in words they understand. Parents give legal consent, but the child's assent is sought too.
- Cascade testing
- Offering testing to relatives step by step, starting with the closest, once the family fault is known.
- Surveillance
- Regular planned checks, such as scans or eye examinations, to find a tumour early in someone who carries a fault.
- True negative
- A negative result when the family fault is already known. It means the sibling did not inherit it.
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Side by side
What does a sibling's result change?
The sibling carries the fault
The sibling does not
Joins a surveillance plan suited to the syndrome
Needs no extra checks for this fault
Tumours, if they come, are more likely to be found early
Their cancer risk is that of any other child
May feel singled out or anxious before scans
May feel guilty or left out of the family's worry
Could pass it on to their own children one day
Cannot pass this fault on
Being straight with you
What this page cannot tell you
It cannot tell you whether your other children should be tested, or when. That depends on the exact syndrome, the exact variant and the age of each child. What your specific variant means is a question for the counsellor who ordered the test.
It cannot predict how each child will take the news
Some children shrug. Some worry quietly for months. A sibling who tests negative can feel guilty that they escaped, and one who tests positive can feel different from their friends. A counsellor or a child psychologist can help, and it is worth asking before the result, not after.
Who this does not apply to
Most brothers and sisters of a child with cancer do not need testing, because most childhood cancers are not inherited. If no fault was found in the child who had cancer, there is nothing specific to test siblings for. Their care is guided by the family history instead.
Commonly believed
Four things families tell us, and what is actually true
"Let us test all the children at once and get it over with."
Testing at once makes sense only for syndromes where checks start in childhood. For adult-onset faults, testing a young child takes away their own choice later and changes nothing now.
"She looks perfectly healthy, so she cannot carry it."
Carriers look and feel like everyone else. The fault causes no symptoms of its own. Only the test can tell a carrier apart from a child who did not inherit it.
"If his test is negative, he never needs a check-up again."
He needs no extra checks for this fault. He still needs the ordinary care any child needs, and any new symptom should be seen like it would be for anyone else.
"Children should not be told why they are being tested."
Children usually sense that something is going on. Simple, honest words suited to their age tend to cause less fear than silence, and they build trust for the checks that may follow.
Questions we are asked
Common questions about testing siblings
At what age can a sibling be tested?
It depends on the syndrome. Where checks start in infancy, testing can be done at birth. Where they start later in childhood, testing is timed just before the checks would begin. For adult-onset faults, it waits until the young person can decide. Your counsellor will tell you which applies.
What if the parents have not been tested yet?
Siblings can still be tested for the family fault directly. Testing the parents first is helpful, because it shows how likely each sibling is to carry it and which relatives to approach next. Many families test the parents and siblings in the same few weeks.
Can a teenager refuse to be tested?
Their view carries real weight. Counsellors will not push a reluctant teenager into a test when waiting does no harm. Where early checks would clearly help, the counsellor will talk it through with the teenager and parents together to reach a decision everyone can live with.
What about a baby we are expecting?
Talk to the counsellor before the birth. For some syndromes the baby can be tested soon after delivery so checks start on time, or stop before they begin. Testing during pregnancy is also possible in some situations, and the counsellor can explain the options.
How do we explain the test to a young child?
Keep it short and true. For example, that doctors want to check whether their body has the same instruction as their brother's, so they can look after them well. Answer questions as they come. The counsellor can suggest words in Telugu that fit your child's age.
Is a sibling's test expensive?
A test for one known fault is usually much cheaper than the first test in the family, because the laboratory checks only one spot. Prices vary by laboratory. Ask the counsellor for a quote and whether any subsidised programme covers relatives.
Should we tell the sibling's school?
There is usually no need to share the result itself. If a child joins a surveillance plan, the class teacher may need to know about regular hospital days. What to share, and with whom, is the family's choice.
Where do we start?
Ask the counsellor who explained the first child's result to talk about the brothers and sisters. List each child's age and any health concerns. If you are unsure who to approach, call the CION helpline and someone will point you to the right clinic.
Your Specialists
Meet CION's oncologists. Bring your family history or genetic report to them.
Our medical oncologists see people with a strong family history of cancer, arrange genetic counselling and testing where it fits, and plan the checks that follow.
Medical Oncologist
Dr. C. Raghavendra Reddy
MBBS(Gold Medal), DNB(General Medicine), DM(Medical Oncology)(Gold Medal)
Medical Oncologist
Dr. Bharati Devi Gorantla
MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
Medical Oncologist
Dr. Owais Mohammed
MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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Sources
- National Cancer Institute — Genetic Testing for Inherited Cancer Susceptibility Syndromes
- NHS — Predictive genetic tests for cancer risk genes
- GeneReviews (NCBI) — Retinoblastoma
- GeneReviews (NCBI) — Li-Fraumeni Syndrome
This page is general information, not a prescription. Do not change or stop any treatment based on what you read here. If anything is worrying you, contact your own treating team — or call our helpline and we will help you reach the right specialist.
Keep reading
Related pages
Testing the parents after a child tests positiveThe result that tells you each sibling's chanceExplaining a genetic result to a childWords that suit your child's ageWhen a child should decide for themselvesFor faults where testing can waitSurveillance programmes for predisposed childrenWhat follows a positive result
Talk to us
Not sure whether your other children need testing?
Tell us the syndrome and the ages of your children. We will help you understand whether testing now would change anything, and arrange counselling if it would. One helpline serves every CION centre.