Tell them honestly and early, using simple age-appropriate words: their brother or sister's brain works differently - it isn't an illness, they didn't cause it and can't catch it - then keep the conversation going. This works whether you have a confirmed diagnosis or just a strong suspicion; the honest version of the conversation is the same either way.
Start with the honest, simple explanation
Pick a calm moment with no rush and no screens, and say it plainly. Frame autism as a difference in how their sibling's brain is wired, not as something wrong or something that needs curing - for example, "Sam's brain works in its own way, so some things that are easy for you feel really hard for him, and some things he finds easy are hard for other people." Match the words to the child's age and keep it concrete. The National Autistic Society's short "What is autism?" video and the CBeebies programme Pablo help younger children picture it.
Give the two reassurances they actually need
Most guides list general tips but skip the two worries young siblings carry silently. Say both out loud, even if your child hasn't asked:
- You didn't cause it. Autism is something a person is born with, to do with how the brain develops. Nothing a brother or sister did, said, or felt made it happen.
- You can't catch it. Autism is not an illness like a cold or flu - you cannot pass it on or catch it from someone.
Children rarely voice these fears, so naming them first lifts a weight they may not have known how to put into words. It also helps to add that their sibling is loved no less, and that needing different support doesn't mean being loved more.
Keep it going, and protect one-to-one time
This is one ongoing conversation, not a single big talk. Invite questions, come back to them as your children grow, and let them know that whatever they feel - sad, jealous, confused, cross - is allowed. Build in regular one-to-one time with each child that is just theirs, and a little dedicated listening time, so the sibling doesn't feel that the autistic child gets all of you.
You don't have to do this alone. In the UK, Sibs is a charity for the brothers and sisters of disabled children, and its YoungSibs service supports siblings aged 7 to 17 directly. The National Autistic Society also publishes a free guide, Family relationships - a guide for siblings of autistic people. For more on the day-to-day, see how to support the siblings of your autistic child.
Where the law comes from
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This page is general information, not clinical or legal advice.