Sibling is not underage and parents are in the picture but myself and they also support her wellbeing. They are currently stretched very thin so I do what I can often to support them and my sibling.
Well, since she's not a minor, the reality of the situation is that your parents really can't do anything to help your sister either. But the good news is that your sister is in therapy, and her therapist is the best person who can help her. So just continue to encourage her to keep going to therapy, and using the DBT tools, which will also help address her abandonment anxiety. Doing enjoyable activities together like going on a "girls' night out" once in a while could also be something to do, if you don't already.
But what stands out to me is that it seems like you may the "overfunctioner" in your family, while everyone else is underfunctioning to one extent or another. Overfunctioning is really just a way to manage anxiety and it is very effective at doing that, but it comes at a high cost. It might seem counter-intuitive, but the best possible way you can help your sister and parents is to begin dialing back on overfunctioning for them so that they can learn to become a bit more self-reliant. Now this will not be easy at first, and you should expect that your will get a fair (or huge) amount of flak for doing so, and will probably be accused of all
manner of things like that you care only about yourself, and that you're abandoning them, being selfish, and if they pull out the big guns, of being abusive, but it's just their own anxiety talking, and with time they will eventually adjust. I've been at this myself for over two years now with my family, and even though at first everyone positively lost their sh*t, I think they are finally accepting defeat and realizing that I will not be going back to my old ways.
If any of this resonates, I can tell about a couple of books that I have really helped me if you're interested.
