WOW!

I don't want to give you a standard answer but this REALLY needs to be handled by a professional.

Although 11 yrs old is a little young, there is nothing wrong with an 11 yr old boy being sexually curious (I am sure you know this.)

The problems arise with how he has 'expressed' his curiosity. You MAY be right about sexual abuse (the hypochondria and the **** entry raise red flags). I would STRONGLY suggest you take BOTH children to a therapist and try to get to the bottom of what is going on.

******I am NOT a professional so, while my advice to seek help should be taken, don't jump to any conclusions about the 'why' of it happening. I don't know your son, the situation and don't want to see you falsely accuse anyone of misconduct.*****