I understand why you feel trapped in this pattern. Your parents would rather make you responsible than accept responsibility for themselves. Being a responsible person, you can’t let things drop. However, it is part of growing up to set boundaries with parents and to separate from destructive family patterns.
Regardless of other issues, it is your right to refuse to talk to your parents about their complaints with each other. Remind them that they got a divorce for good reasons and there is no point in continuing their fight with each other through you. Then change the subject. If they won’t change the subject, tell them a polite “good bye” with the comment that you’ll be glad to hear from them when they want to talk about something else.
I am aware that some of the things your parents want you to deal with are too important to abandon without putting something in place. For example: You didn’t mention how old your sister is. If she is a teen, it’s time for her to take responsibility for managing her disease since her parents can’t or won’t. Talk with her about how she is coping with the self-discipline that is required for good diabetes management. Encourage her to have a talk with her endocrinologist and perhaps to seek out a support group for young people who are dealing with diabetes. If necessary, remind her that refusing to take care of herself is not at all likely to get her parents to change. It really is up to her to do what she needs to do to have a long and happy life.
As for things like your financial aid: Do talk with the Financial Aid office at your school to see if there is any way they can get you out of the middle by dealing with your parents directly.
If you find that you just can’t extricate yourself, please do consider seeing a therapist for support.
I wish you well.