This is a tough one! If they specifically told your niece that YOU might do these things, there are two possibilities:
One, maybe they don't fully trust you and it makes them uncomfortable that she jumps on you, so they told her this.
Two, maybe they are very concerned about sexual predators and are trying to let her know that these kinds of things happen.
Either way, I believe they went about it the wrong way. If they don't like her jumping on you, they should discourage her from doing that, and let you know that they prefer no (or not so much) physical touch. To specifically name you as a potential predator as a way to buffer her is inappropriate IMO. Great to tell kids that anyone touching them in the wrong way is never ok, no matter who it is. I created an awareness for my daughter, but didn't turn her against specific people.
We are supposed to teach children to trust their gut. To name you as someone to avoid without cause will only serve to make this child mistrust her own intuition, this is a problem!
What they should do, if they are concerned (in general or for a specific reason, which is certainly their prerogative) then they should supervise her, or avoid you without calling you out.
My FIL used to kiss my daughter all the time, she didn't like it and I didn't either. He is the greatest guy, just made us a little uncomfortable. Never did I say anything to my daughter, other than 'you never have to kiss anyone you don't want to'. We just explained to my FIL that we're not cool with this, not that he did anything wrong, just stop. He did and that was that.
Obviously, if they have a specific reason to want to avoid you, I hope it comes to light to be addressed. And I know you have your niece's safety in mind. Hard not to take things personally, I'm sure. Your wife needs to be on your side, but no one can change these people. So even though I explained why what they are doing is probably more harmful than good, it is how they are.
I'm not sure why this is destroying your marriage. You need to ultimately make sure things are right with your family. Once that is secure, you can include other people. Don't punish your niece, and don't grand-stand by declaring you will never visit them. This is their issue, if you get right with your wife and you family, the rest can work itself out better.