<human>: Could you distill the essence of the text into 1-3 sentences?

Using a throwaway. I have no idea what to do. my niece is 3 and my nephew is 4 months. My sister in law is very unstable. I know she doesn't take any medication for her ADHD. I think she's also either bipolar, manic depressive, or has narcisstic personality disorder. She can't stay in one place for more than a month or two. She's with a different guy every other month. She changes jobs all the time.

The father of her oldest, the 3 year old, is not in the picture. He's a registered sex offender and is in and out of prison. The father of the 4 month old is a good guy, a hard worker, but he doesn't put up with her shit anymore. 

My sister in law's most recent ex, IDK anything about him but he's saying all over facebook and social media that she openly beats her children. I've seen my niece fairly recently and I didn't see any bruising on her. My husband [29M] saw her yesterday and said there was no bruising either.

I worry because these are young children and their mother doesn't seem to care about their wellbeing. A year or so ago we babysat the niece and she ended up giving us bedbugs. She used to live with people who did drugs or sold drugs because they weren't charging her for rent. 

She lives an hour away from us so I'm not sure that calling the police or something would do any good. We don't have any evidence. I just feel so bad for these children. They don't deserve anything that could be happening to them. My niece is always so happy and wanting to play. My nephew is so young, he doesn't know anything yet. Idk what to do. Any sort of advice is welcome. I know I probably shouldn't really do anything because I have no evidence, I don't really talk to her, and this could come back to haunt me later.
<bot>: My SIL might be beating/abusing her young children. Don't know what/if I can do anything. Advice?