Selling the car is a goof idea. Not sure what their plans are with that or if whether or not it has even been discussed at this point. I'm the only one saying she doesn't need to lawyer up unless the lady dies. However even with all her ducks in a row and the police telling her it seems like a suicide she has an attorney already just to be, safe. I think it's unnecessary at this point but I'm alone in that.
...and never rely on insurance company representation.
Do consider the woman's possible assets for a lawsuit. It is a valid thing to think about, especially regarding an illegal act. Public consumption, public drunkedness, walking in a roadway, suicide is even illegal in places... Whatever charges you can get the cops to officially note/charge is important. This person's estate/life insurance/homeowners insurance, etc. should have a claim filed with proceeds going to making your MIL whole again. Car, therapy, time, effort, gas to therapy sessions, etc...
Protection of MIL starts with the little things (i.e. a citation for jaywalking) for the legal protection MIL needs from a great niece of the suicider who wants to sue MIL.
Seriously. Trust me, in some states this is a lifesaver... True in car accidents too. Get the cops to give the other party a ticket for anything, it's gold.
PS, never just think the person is indigent. Insanity isn't just for the poor. Do a due diligence asset search.