My mother is presently in a situation like this. I was fortunate enough extricate myself from the mess by emmigrating to Japan, but my mom is stuck. Her sister, you see, only creeps in for financial assistance or booze money. The squalor my aunt lives in is her fault though. When my grandmom and mom try to help and be a family, my aunt disappears and won't answer phone calls. My mom keeps asking me what to do, how to act in certain situations, but I honestly don't know how to advise her. Even though I tell her that it's okay to snap, to throw up her arms and say 'eff this crap', we remind each other that we're dealing with family.
Eh...maybe I never really left that mess either haha. It's definitely not simple to say when enough is enough. Only in time, I suppose.