4 year old humping 3 year old?

Thank you all for the posts! I feel much better about it today. I did a lot of reading and talking and searching. I believe she was either sexually abused or wittnessed something in her first foster home. They were planning on adopting her when she was removed due to their other pre teen adoptive children (brother and sister) were caught having sex with eachother. They "say" that nothing happened to her but I doubt if the bro and sis were left alone long enough to do what they did ...that they were watching her as closely as they should have been. I don't know ...I wasn't there. That is half of the problem, not knowing. My Mom and I gently talked to her today and went through her life book to see if anyone triggered a response (verbal or non verbal) and she hesitated on the girl's picture but said she did not remember her. She told us later that our niece had done something to her in a bedroom but we know it isn't true because she has only seen her twice (she lives out of state) and has never been left alone with her. We have always spent time in common areas and doors are not closed in my mother's house. After much thought I looked at her book again and saw what a HUGE resemblance the pre teen from the first house looked like my niece. They could be sisters. I'm probably just grasping at straws but it is the only thing I have right now. As for my time away from the house I was down the street picking up food for dinner. I was only about 4minutes away and only gone for 15 minutes. As for my husband...this was the first time for him wittnessing anything like this. He wanted my opinion before making any decisions. We share parenting. I would have done the same with him. He didn't freak out...I did. I think I was disgusted because she hurt someone I love that was innocent. Our 3 year old has no interest in anything like this. She pretty much sees body parts just as she would treat a shoe...its just another thing. If and probably when the 3 year old goes back to her bio Mom she is going to have enough issues to deal with. I don't want her to have this one too. Just wanted to keep her safe. Talked with "I" 4 year old today and told her the same speech again but this time I said that she should have something to do when she feels the urge to self please. Yes, I used terms she could understand. All, I could think of was a binky (I know 4 is too old but it much better than humping something) or maybe a soft blanket or something. Any ideas on this? She promptly refused the binky anyway. Another problem is that we are not bonded very well. I care for her, I love her but I do not feel the same connection that I feel with the 3 yr old. I want to so badly but I think I am trying too hard. Someone said "fake it till you make it" I have been trying that too. She is so smart and funny and she has overcame almost every obstacle she has had since we have had her. She has one still hanging on .... I just want to help her. So, we will see what counseling has to offer and keep at it. Thanks again for all the words of comfort and even the words to jolt me. I needed them too.