I have always wondered how brother & sisters acted sexually when they are latch key children without adult supervision. So sound your horn. Here is some primier from Wiki.
Childhood sibling–sibling incest is considered to be widespread but rarely reported. It is believed to be the most common form of intrafamilial abuse.
Sibling-sibling incest becomes child-on-child sexual abuse when it occurs without consent, without equality, or as a result of coercion. The most commonly reported form of abusive sibling incest is abuse of a younger sibling by an older sibling. A 2006 study showed a large portion of adults who experienced sibling incest abuse have distorted or disturbed beliefs (such as that the act was "normal") both about their own experience and the subject of sexual abuse in general.
Sibling abusive incest is most prevalent in families where one or both parents are often absent or emotionally unavailable, with the abusive siblings using incest as a way to assert their power over a weaker sibling. Absence of the father in particular has been found to be a significant element of most cases of sexual abuse of female children by a brother. The damaging effects on both childhood development and adult symptoms resulting from brother–sister sexual abuse are similar to the effects of father–daughter, including substance abuse, depression, suicidality, and eating disorders.