Children Undressing Eachother

Alright guys I need some help here. My SIL has been having some trouble with her son. He's 6 years old. My SIL is a single mom with 2 kids, RJ and KJ (2 years old girl). RJ has been acting up, taking toys from other kids at school, showing some aggression, being jealous of his sister, etc. My SIL has been having personal issues besides her son so she asked my MIL if he can stay with her for a little while so she can get a break and sort out her problem. Well my other SIL has children from 6-13 so she took in RJ for a month.

Forward to 2 weeks in to his visit, RJ was playing with his cousins and at some point took his youngest cousin Althea to the book nook (hall closet size without the door). My other SIL went to check on them and saw they were both undressed. She got mad, they got punished, then when she confronted him he claims he saw "guys sucking on girls titties on her tv". Little back story, my SIL never watched anything over PG while the kids are in the house so that was bogus. His mother wasn't hardly concerned at all about what happened.

So my husband and I were talking about it after his mom told him what happened, given this happened on Monday, and we don't know what to do about it. My husband was completely sickened by it. We think it might be some sort of trauma he has from his mom, not in a sexual way but in a separation way. She went to jail for a few months when he was 2. Nothing major. My SIL basically doesn't know what to do with her son either and we'd probably have to tell her what to do or where to get help and so forth.

Are we thinking too much into this or is there something we should look into doing, such as counseling or therapy or some sort of boot camp? Help please! Thank you and sorry this is so long.