how to set boundaries?

[fevredream]

me again.  My sister had her baby at 34 weeks.  The baby was born with some serious chromosomal abnormailites that resulted in her dying at 2 weeks old.  My sister was devastated.  Baby was buried the weekend after Thanksgiving.  I have been still paying her bills.  found her another car that a friend gave me for free.
last week she told me her friend's toddler broke her tooth.  this weekend she went drinking at her neighbors house and when she went home (shared driveway) she slid into her apartment and busted up her car and her apartment.  the house is unlivable at this point.  She hit it right where the gas, water and electricity enter the building.  it tore her car all up.  She begged to go to the hospital and be admitted to the mental hospital.  They had her speak with a crisis counselor who sent her home and told her to see her primary doctor about a change in the anti depressants she is on.  and then my sister called me and told me that it was actually her baby dad who threw something at her or hit her or pushed her and that is how she broke her tooth.  I hate him.  He didn't even bother to come to the funeral of his child.
I know I can't keep saving her from herself and her choices.  but I don't see any way for her to get the help she needs unless she has a car.  We live in a very poor rural area.  there is no option for public transport.  She lives 10 miles outside our small town.  the only way she can get back and forth to doctors or a job or anything is the have a car or have friends that will drive out to her to give her a ride.  People say I have to cut her off and quit helping her and she will get better and be able to help herself.  but I don't see how.  it all seems so messed up.
how can I set boundaries when i feel like I have no choice but to keep helping her?  if I don't help her, what will happen to her?  and there is the pea soup FOG I am in.Why don't I feel like I have a choice?  What happens to her if I withdraw all my support?