but why does cos need to be involved? I understand sibling abuse, but is it really necessary?

For safety reasons, if the instigator knows that the behavior is inappropriate and pressured the other sibling into it. With sibling abuse, they will usually put the instigator in an alternative home- either a foster family, group home, or families/faculties that specialize in handling cases like it- while both children receive very intensive therapy. If the abuser can be rehabilitated to be safe around their sibling and other children they can be allowed to move back in, if not, the parent is encouraged to stick with the alternative housing in order to keep the victim out of the abusers reach. CPS really only needs to be involved if OP needs help handling the situation- which would be understandable.

OP should just stay calm and try to not overreact. Since the children are twelve years old chances are good they only did some experimenting. If the act was not voluntarily but the result of any form of abuse I still suggest OP and the father try to separate them and protect the abused one without involving the authorities, even though police and CPS could be used as some sort of final thread. "Do as we tell you or we have to move you out od this house!" I’ve simply read and heard too many stories of CPS mishandling cases. OP shouldn’t give up on a twelve year old child too early. A twenty year old abuser might be a permanent thread but at that young age it’s highly unlikely that an abusive child understands the full magnitude of his or her actions.

By failing to involve the proper authorities, the parents would be endangering the abused child and teaching the abuser that there are no real consequences for abuse. They have to stop the cycle, and both kids definitely need professional help.

Abused by the people who are closest to you and who you share your living space with no choice is very traumatic. To prevent such situation it is may very likely be necessary