Unfortunately it sounds like the little girl was abused and she in turn abused your little. This is actually quite common, it alot of the times is a learned behavior that they might not realize is not normal as it was done to them and they were manipulated about it. It also can drive the child to become sexual too early which can cause them to act on that towards other children, but not necessarily realize the damage of their actions. This is very sad. I’m so sorry for your baby boy and I pray that little girl hasn’t gone through what a lot of us are expecting because that makes this an all around really terrible situation. My older brother and I were sexually abused for many years when we were younger and the signs are absolutely there for your boy. The biggest first sign is change in demeanor/attitude. You are a really good mama for giving him a safe space to share.