Hyperbole really isn't going to help OP.
Lots of 13/14 yr old girls date 15/16 yr old boys - it doesn't make the 15/16 yr old a paedophile.
Obviously he sexually assaulted his sister which is horrific. But claiming he's a paedophile and attracted to children as an adult is a huge leap. OP says he was 15/16 at the time, a child himself. There's nothing in the facts presented that suggest he has paedophile tendencies. Sexual assault, yes, but paedophile, no. Let's not obscure the facts here.
Mary Bell was a child killer, killing other children when she was a child herself. She's gone on to lead an entirely uneventful life with no other violent crime.
It is entirely possible that whatever was going on with him at the time is no longer an issue now, but I think it needs to be explored professionally. It's left his sister with trauma, and I can't imagine how you face a partner who's confessed to that, and continue to feel the same way about them.
Professional help to work through this is essential, for the sake of all involved. This is a huge and shocking revelation, and of course you're going to want to seek assurances OP that there's been no other sexual deviance.
But I agree with PP that with an immature teen-child mind it's possible to be a person and do things that would be unthinkable as an adult. This crosses a lot further than most of us could imagine, but the principle remains the same.
A level head is needed here to figure out what you want, what you can accept, and how to move forward - with or without him. And I think you need professional help to do all of that. All the best OP.