I've just read through this and can't really add to all the advice so far. But, I'll say...based on your latest response...maybe you need to say no sometimes. There's nothing wrong with being a bit selfish so that you can take CARE of yourself.
I'm glad you've cleared the air with one sister, I hope you can all be in tune with one another soon. It's important for families to join forces when things are tough. Best wishes to you and your family.