Husband molested sister when kids

Hi everyone. I am trying to understand my husband told me and I am having a real problemm with it.
I guess Ill start from the begining. My husbands sister (now 28) was dating a man for 18 months and she was supposed to come to visit her family and friends on her own. The day before she flew out they decided he would come too. My inlaws got a phone call from him saying that he planned to propose while they were visiting. My inlaws said they did not give him their blessing (felt he was controlling, has 3 kids that my sister in law was looking after, she has to work 3 jobs while he works part time etc). They agreed to all talk when they flew in. At this point I told my inlaws I would make myself scarce while they talked and they said that they wanted me there.
The meeting didnt go too well. It was like listening to 15 year olds 'But I love him!' and 'We are gonna be together forever'. Also during the conversation the guy said 'I created her and I made her who she is'. That was the tell tale sign he was a narcisist. Anyway, for the next 18 months my sister in law kept falling out with my in laws, they were going through hell and I stuck by their side and couldnt understand what was wrong with her and why she was choosing this guy over her own family!
so 18 months after the proposal, my sister i law visits and we get together for a family game of cards. We all got completely hammered and the whole game, every 2 minutes she is picking up her phone and sending messages to her fiance.
On the way home I am going off on one about the fact she couldnt even give her family a weekend without having to text this guy every 2 minutes. My husband looks like his head is about to explode. He screams that 'when they were kids I came on to my sister', I asked him what that even meant and he just closed up. I completely went off. I was throwing things and called him a paodophile.
Turns out he was 10 and she was 6.
I just dont understand. I have never heard of this before. Flashing is one thing, or the' ill show you mine, you show me yours' but this was actual touching.
Now hes mad at me for my reaction, and I dont see how. It wasnt like he sat me down and explained this to me, I have never reacted to anything like that before, but quite honestly it disgusted me.
I am also so mad at his parents. Before she was with the narcisist she was with a physical abuser. I feel like I have been a pawn in some sick game because instead of fighting and falling out with their daughter they should have been talking to her and getting her to see a councellor because im sure this has something to do with why she cant have a healthy relationship. I feel bad for the things I have thought about her and how Ive just blindly supported my inlaws.
We are currently arranging to see a councellor (me and my husband) but he wont talk to me about anything
Can anyone help with this?

Really difficult situation so i'm glad that you're in the process of waiting for counselling at the moment.. that's the absolute best thing that you could do here.

Usually.. with children and whether or not it was abuse, the general thought is that when there's an age gap of over 3 years, it is abuse because the older child is at a completely different developmental stage than the younger one and could not be thought of as ''peers''..
However.. a 10 year old is still really really young and being a parent myself - I know just how little most 10 year olds would understand about the rights and wrongs and consequences.

What I can see from your post is just how guilty your husband currently feels and It's likely that his deepest fear was to have you react the way that you did.
The way that your sister in law has behaved throughout her adult life however is indicative of a history of abuse so it would be helpful to understand exactly what happened between your husband and his sister as well as knowing whether your husbands parents were abusive in any way..

The fact that your husband was too young to really understand the ramifications of his behaviour doesn't take away the impact that it would have had on his sister throughout her life and therapy for his sister sounds like it would be incredibly useful as well if she'd try that.

Thank you so much for your response!
I understand he feels guilty, and that I didnt respond well but I was not in control of my reactions in the way that he told me. We have been together for 4 and a half years and as far as I knew we were completely honest with each other and trusted each other. I should have been told, and I mean sat me down and explained it to me, either before we were married or at least when all the stuff with his sister and parents started. It should not have been shouted at me in anger and then bottled up with no other information leaving my mind to come to its own conclusions.

We had an amazing relationship before this. Now, while waiting for the councelling to be set up, we hardly talk. He wont talk to me about ANYTHING. It is like he is emotionally void. I am losing a lot of respect for him and think the honest and trusting relationship we shared may have never even existed.