Listen up If I were you I'd talk to him about first. Because the information his family has given could be false. Depending on the answer will depend on what choice your going to take.
If you still don't trust him then you need to either split up with him if you don't want to do that then make sure you're daughter is always somewhere you can keep a eye on her. Then move away from him as soon as possible I wouldn't even take chances since my daughter would be in danger and since she's so small he could do something to her that could even end up with her being dead or when she gets older she might be scared with memories. But then again he was only fourteen when he did it so he might have not been fully concious of what he was doing since he's an adult now he knows what's wrong and what's right also as someone said 'people at that age like to experiment'.
I'm not trying to scare you but I'm trying to warn you also make sure to tell someone close to you about this just in case he tries to do anything funny so you always have someone who knows about the situation. You should look forward to the future not looking back at the past, and haven't you know him for 6 six years? Since you've known him for such a long period of time then you should be able to tell when he's lying or not and if he's the type of person do that.
Well if his sister wanted to "protect" him then she wouldn't be telling you this and would let him date the person he's in love with so there is something obviously wrong with that as well. Don't accuse him straight away he might be denying it because he actually didn't do it. You might even be thinking to deep into it - that might explain the reason why you thought that she'd want him if she didn't have a boyfriend.
Don't ask his family about it at all you should listen to the truth from him only. If he has been molested as a kid and still can remember than I doubt that he did it to someone else and his niece might be making everything up. I hope things work out of for you.
Gee! He was 14 years old when it happened! Lots of kids experiment at this age.
Also look a the role models he lived with. If they are all incestuous why did they make such a song and dance. Did they not think the behaviour was normal?
I expect he's grown up since then. Ask him about it (what did he do) before you make any decisions.
I cant really gove you an answer,but what I can give you is a way to a solution, that is you have to find the anglde that you relate to or peaks your interest. A good paper is one that people get drawn into because it reaches them ln some way.As for me WW11 to me, I think of the holocaust and the effect it had on the survivors, their families and those who stood by and did nothing until it was too late.