I think the key to dealing with crazy-butt relatives is that you have to take care of yourself first. No matter how much they think you're helping, or no matter how much you want to help, if you're sacrificing your sanity for their sake, that's no bueno.
So the only real qualifications that I have on the topic is that I've been blessed with two crazy-butt relatives. Good times! I used to get pushed around quite a bit, but then I grew some cajones & had a family (and my sanity) to protect. There came a point that, as soon as I knew their children were safe, I had to say, "No more, Crazy-Butt!" That meant dealing with tantrums, threats of violence & name-calling. And a lot of emotional hurt in the short-run that goes along with that, but ultimately it has been way more liberating & healthy to keep the crazies away.
What if you still want to keep a relationship with Crazy McCrazy-Butt, but feel like you're being taken advantage of? There are lots of polite, non-dramatic ways of saying, "Hey back-off Crazy-Butt McGee!" Something like, "I'd love to help out, but right now I need to keep my schedule open." Or if you're not into the white-lie thing (because let's face it, if you're trying to get out of something & don't have a legitimate reason, you probably wouldn't really "love to help out"), then when the favor of, "Can you __(watch my kids/ pay my rent/ give me a ride/ loan me some cash)__?" is asked, then I think it's perfectly okay to simply say, "No." Or even, "Not right now." "Not right now," should be a pretty easy thing to hear. It leaves you open to help in the future, but not obligated. And if they really can't handle that answer, then it's okay to just let them deal.
If you're dealing with someone whom you fear might be a danger to you, others, or themselves, call your local authorities right away. Especially if children are involved.
Want advice from a redhead? Ask in the comments section & I'll totally give you my advice. I can spout off potentially sound advice on almost any topic. Except math.