@4thtwin,
The best way to keep your siblings from being all up in your business if you inherit the car
is for your parents to have a written will, drafted with the assistance of an attorney, notarized, the whole shebang.
That way, if they end up complaining, you and the probate court can tell them to go pound sand.
As for paying for a roof over your parents' heads, etc. Your sister has probably realized that if the parental home is in disrepair, it'll become unsafe. And your folks will potentially be better off in assisted or independent living (both of which are very costly no matter where you are) or with aides (also costly). Or, your folks will end up living with one of their children. Your sister's probably worried that it'll be her.
So,
if you wish to maintain ties with your family, make a choice on what you think you may want to help cover. Because I can more or less guarantee that your folks are going to need some form of help, and that it will be costly. If you want to keep your familial relationships, then recognize that you're paying now, or you're paying later (aging in the US is the ultimate money pit). But if you don't care about keeping a relationship with your family, then feel free to wash your hands of it. I can't speak for whether you'll feel guilty, or if they will judge you. You might, and they probably will. So, if you go that route, understand what it entails.
Also, not for nothing, but if your mother is in her 80s, then she probably shouldn't be your dad's caregiver anymore. Are either of your brothers who live there helping her? If they are, then great! If not, then you and your siblings should probably have what in the world of work is often called a "come to Jesus meeting", where all of you inform the brothers that party time is over, and your parents need help. They can either step up and physically help, or they can help pay for it. The latter undoubtedly means they'll need to get jobs.
Why am I saying this? Because caregiving is stressful, and if your mother keeps this up, she is punching a one-way ticket to a stress-related disease or injury, like a heart attack or stroke or a hip fracture if she falls while trying to care for him.
Regardless of how you may feel about your siblings or about your own personal financial situation, I would strongly suggest that you at least find a way to help with getting your mother some relief. Because if you (and I mean all of you siblings, not just you, personally) don't, then you won't be dealing with just one parent who needs care.
You'll be dealing with two.