is this sexual abuse

It very well could have been abuse. Based on what you've written I'd say it was.

It would be unusual for a 12 year old girl to have an "awful smell" so chances are she was being abused as well.

Your husband should speak to a therapist and work through his thoughts and memories about this.

It is not unusual for young friends or siblings to be put into situations where the "you show me yours, I'll show you mine" thing happens.

On the other hand, if he was repeatedly forced to do this, it probably was abuse.

Abusers look for the "weak link" and it sounds like she knew that. Is he still in that role?

Therapy would help him become more assertive and give him the ability to verbalize and realize what that sibling interaction was really all about.

Thanks for your prompt reply. Yes, he is still the weakest link. Easily manipulated as well. He is also very insecure and he doesn't feel great about himself despite the fact that he has a great paying job, a very nice looking body, handsome face, well endowed, etc... all the ingredients a man you may think would need to feel good about himself and... he is also very intelligent, yet his personality doesn't match. I have thought that it is because he was abused as a child and he has blanked it out. Also his father is very dominant and his mother is a floor mat! She is very submissive, his father is chauvinist, the kind that "the woman stays at home and the man works". My husband has some of those tendencies to be chauvinistic but as I am a very strong person (by the way, that is who he is attracted to) just find it funny and hilarious and laugh about it. He can't try that with me anyway. I am independent, self assured, very high self esteem and that also seems to bother him a lot, yet, he is attracted to me for it!? really weird! He also has problems with intimacy and he is not confident in the bedroom at all. All to which gets me to the conclusion that he must have been abused as a child. I have suspected his father but he has no memories of it and barely remember details of his sister's situation...

It just might be his temperament. (low key, shy, introvert, passive, sexually inexperienced) Don't assume he has been "abused"

It sounds like you have enough energy and self confidence for the two of you.

Let him be and enjoy him as he is - because that's WHAT he is.

I am not assuming, he told me and he seems to think what went on with his sister and him was not only child's play. I don't mind him being the way he is. That is why I like him for being him, but also it is not the fact that he is shy and low key, it is his self esteem issues...