My kid's mother has a niece who is doing onlyfans, do I look at it?

Eh, this is tough. I mean it's not like it's family or even step family; if that were the case then I'd have to drop an easy NOPE. But this is some grey area, and probably depends on how you want to proceed with interacting with your ex and her niece. It's totally different when it's someone that you don't know, but the fact that you know the niece makes it a bit more of a slippery slope. Imo, I don't think I'd do it, but I mean if you don't really care about further interaction with your ex, then what the hell, go for it.