Welcome. Everyone's questions are wonderful. The answers will help us help you.
I have more detailed questions about the sexual abuse. in my opinion, when we live in chaos with a difficult child, we start having knee jerk reactions to EVERYTHING when in fact not everything is or is part of the problem.
I can't think of a gentler way to ask, so I'll just be direct. Did he really ABUSE his siblings, and continue to do so when told to stop? Or could it have been curiosity and exploration? Between the ages of 4 and 8 kids discover that boys are different from girls. There is a natural curiosity that often leads to exploration (playing Dr). If children are never told that this is inappropriate, then they just don't know any better. The curiosity kicks in again at puberty, so if the initial lesson of that behavior being inappropriate isn't learned, then it can repeat itself.
It is during this age that natural modesty also kicks in. My son hit modesty at age 5, DD1 at age 6-7 and DD2 still has not fully formed it. She won't run naked in front of strangers, but has no issue with running around in just her panties at home - this bothers her older brother TREMENDOUSLY, so we make her cover up. Does your son display any modesty?