No, Tupps - it's not selfish to have an instinct for self-preservation... to take care of you and yours first... it's a responsibility to yourself, to not spread yourself too thin. It's normal to wonder if giving help will actually, truly help... and to be a bit a reluctant to open yourself to expectations of more, more, more - sans clear boundaries.
Your sister needs to find her own motivation, get up and go, to help herself and figure out for herself what she needs, wants and go get it. I struggle with this, too - with my daughters. I have to constantly restrain myself from riding to the rescue because I have finally seen, that ultimately the message in that rescue is - I don't believe you can do this for yourself, so I'm going to do it for you. That contradicts what I really want - which is for them to develop the confidence in themselves that they can make good choices and take care of themselves. They really don't want Mommy hovering over them, helping, doing for them. One D in particular, judges herself way too harshly for actually needing help from time to time. Apple doesn't fall far from the tree...
What they DO want however, is to know that I care about their trials and tribulations. They do want the security of knowing that I want to help... but that I'm letting go of that process to enable them to spread their own wings, stand up for themselves, and will comfort them the best I can when it doesn't go the way they hope it will.
By standing aside, compassionately, you are helping your sister face, come to grips with, and begin to deal with her own responsibilities herself. That's not something one person can actually "give" to another - you can only point out the path (and only when asked) and then get out of their way.
That said, what Muffin said about normal families helping each other out - I tend to lean that direction too. And I want to help... but what I've learned the hard way is to wait until I'm asked - and then make sure both of us know clearly what's being asked for and to take a day or two to think about how I feel about it - to ask my own intuition whether I'm just being used or if I'm enabling, whether I can ask for a result from my help... whether that's a realistic expectation. And with my Ds, it's finally now possible to talk openly about that and also to explain my own limitations... because they are there and there are very real consequences of my "help" for me; there is always an impact on me. And the emotional ones are the most "expensive", I find.