Hello, spacecase. Why do you think the rest of the family would fall apart if you left? While I commend you for your concern about your family, I wonder why you think it is your responsibility to take care of them?

My thought is for you to talk to sister to let her know how much you will miss her. I do not think it is appropriate to ask her to stay. Maybe you can agree to keep in close connect while apart. Perhaps you might even decide to go with her.

Good luck.