So much to say! I may no present as such outwardly, but I'm generally a compassionate person and really just a big old softie. However...
People do what works for them. Your relative may very well have some mental health issues. She may have had a shit childhood, but there comes a time when those things are no longer reasons, but excuses. She chooses not to help herself because everyone else will just give her stuff.
People gave her vehicles? Damn, I've been doing things wrong all my life. I bought them, with money I earned. And we're not talking new, off the lot here.
I get the guilt. I had to do this with my son. Thankfully, he has no children, it's just him. He experienced some stuff in childhood and I did "all the right things." He also has mental health and alcohol issues he refused to address. He's had jobs, good ones, he's brilliant, but my guilt kept bailing him out. Until I didn't. He sent me an angry, drunk barrage of abusive texts and it hit me..what a doormat I was. He apologized 2 days later because he was out of cigarettes and I said no! I also sent him a long, direct email about what he needed to do. Funny thing, he got a job, went back to AA and seemed to figure his shit out.
I know what you're doing...you're worried about the kid. The kid will be fine. The kid will get what it needs. The skilled manipulator that is her mother wull see to that, with or without you.