I got divorced in the state of Washington, so I'm not sure how much will apply, but I do know that what I did there is also easily do-able here in NY, so, hopefully, some of this is useful.
First, I talked with my local YWCA...they had a lawyer and several paralegals that were as helpful as all get out with filling out all of the forms. (I swear, even tho legal documents are written in a language that looks so much like English, they're pretty inscrutable.). As a matter of fact, when I finally went to court, one of the para-legals went with me for moral support. She was great.
After seeing the people at the YWCA, I went to the court house and talked to the family law facilitators(FLF). Since so many people are getting divorced these days....most courthouses have FLF's to help people who don't have lawyers navigate the system. Most require that you have an appointment...I didn't have one, so I waited in front of the guys office until someone didn't show up, and then begged for that time. He gave it to me and was quite helpful in printing out all the forms I'd need for divorce, child support, etc. He also suggested I see the Domestic Violence judge that morning...so I did.
Speaking of the Domestic Violence judge...My ex never raised a hand to me. He shoved me around some, locked me out of our house...you know, all the things that pussy guys do...but he never actually hit me. But the night before I filed he said "I wish you were a guy so I could beat the living shit out of you". That was enough for the judge to feel that I was in danger of physical harm and serve an order of protection against him. Maybe your BIL's outburst would be the same for your sis.
All of this happened in one day...Order of protection, child support order, divorce proceedings....and the gravy was that because the order of protection had to be served by the cops...I didn't even have to pay to get him served for the other documents....the nice officers that showed up to vacate him from my house served everything for me.



I didn't even have to pay to file...the FLF had me see another judge that waived the fee for me. In the end the whole thing cost me 10 bucks. Best ten bucks I've ever spent. I lost 225 ugly pounds for a mere 10 dollars...
Best of luck to your sister....this part is the hard part for sure....
As far as the debt goes....bankruptcy was invented for just this sort of thing. I was desperately broke in the beginning...but I had to weigh my living situation against how much I was going to pay for the psychotherapy involved if I didn't get my daughter out ASAP. It was tough...but we made it and it's the best, most empowering thing I've ever done.