I am aware this article, even the title, may be upsetting for some, but hope you dont mind that I have posted to make others aware of the seriousness of this. 
Siblings are responsible for more sexual abuse of children than any other family member; the studies indicate that a child is three to five times more likely to be abused by an older sibling than by a father or stepfather. Victims are usually younger than children abused by adults — typically five to seven years old — and abused for longer, on average for five years. The world’s largest study, conducted over two years by Rape Crisis, concluded in 2022 that up to 5 per cent of children in Britain have sexually abused a sibling.
SSA is very different from normal developmental curiosity, or what might be called “playing doctors and nurses”. Definitions vary, but a widely used one, by John Caffaro, an American psychology professor, is “sexual acts initiated by one sibling without the other’s consent, by use of force or coercion, or where there is a power differential between the siblings”. The majority of victims are girls, though the older sibling can sometimes be a sister. The abuse does not necessarily involve penetration, but the most common form of SSA recorded by the police in 2022 was rape.
Dynamics in the families where it occurs often include parental absence or lack of supervision, a family crisis such as parental illness, or parents asking their older child to babysit, oblivious to the risk. Poor sexual boundaries within the family, or exposure to pornography or domestic violence, increase the risk, reports the Lucy Faithfull Foundation, a charity that works to prevent child sexual abuse. Jealousy and resentment within step-families can also lead to SSA, and autism features disproportionately among siblings who sexually abuse. But it can and does occur in any kind of home. Most of the adult survivors interviewed for this piece told me if I had lived next door, I’d have thought theirs was a loving, happy family.
Public lack of awareness of something so common has several explanations. The average age at which a victim first discloses the abuse is 40 — and many never do. Fear of destroying their family silences children; so does fear of being disbelieved. When a child does disclose, parents often find it so unthinkable that they accuse their child of lying, or so unbearable that they — in the jargon — “minimise” and dismiss it as exaggeration.
NB this is a very long article with some distressing detail https://www.thetimes.co.uk/article/revealed-the-hidden-crime-of-sibling-sexual-abuse-z05cf0l8r
I should have said it is written by Decca Aitkenhead. Can also be read at https://archive.ph/RjuYi