Would you interfere in this relationship? (see details)

I have a niece via my extended in-law family. I’ve seen her grow up from a teenager into a young adult, and we’ve gotten along well. She is now engaged to marry a much older man who is only marginally employed, and engages in behavior of questionable legality. Her family does not approve of him, but I’ve tried to be the “cool aunt” so both my niece and her guy are on my Facebook friends list.

I wanted to give him a chance, because there’s a soft spot in my heart for some kinds of social outcasts. However, this guy has recently posted a bunch of pictures of himself in Nazi uniforms, and pictures of his skinhead and Nazi tattoos. In my estimation, he has dropped down many all notches of respect. I suspect if her family knew, he’d become persona non grata in their lives. My question: should I somehow communicate this new finding to her parents or interfere in any other way?

On one hand, I don’t particularly think that he’s the best guy in the world for her: I like her a lot and this guy is trouble. (But they’re in loooove.) On the other hand, if her parents (or even me, the cool aunt) go overboard in our disapproval of this guy, that might make her cling to him more tightly, in a “me and you against the world that doesn’t understand us!” kind of way.

What should I do? My niece and I are not especially close, and only interact at family gatherings, but she’s a nice young woman and I want the best for her.