Am I overreacting or is this gross?

Am I the only one who finds this weird? My ex-husband (39) and I recently filed for divorce 3 months ago, and ever since, he’s been hanging out with my 25-year-old niece—drinking, getting high, and even spending time with our kids like she’s their new mom. Before the divorce, they never spent time together like this. What’s even stranger? He has a niece the same age on his side, but he’s never had this kind of relationship with her.

I can’t shake the feeling that this is inappropriate—maybe even predatory. Is it just me, or does this seem off to anyone else? Would you be okay with this if it were your family?