...but I'll add my 2 cents!
And I have to say I agree with a writer above who suggests to chose who to let into your space/life! I love that we actually get to choose, unlike women in some unfortunate circumstances!
1. Decide on your boundaries and communicate them
2. Take responsibility for your feelings and communicate them to the people who matter. For example, it sounds like there is love between you & your currently flaky sister so if you share with her via phone or email how you feel ("I'm really hurt that you haven't called, I'm really feeling abandoned by you and thought I'd hear from you more. I'm so frustrated in being on bedrest and not hearing from you that it makes me feel like not even talking to you anymore, and that's not how I want things between us... etc.") and ask for what you want ("It would mean a lot to me to for you to call me often and check in on me"), you a)might get through to her and get the response you'd like or b)confirm that she's in a self-centered state and accept that and let it go. Be honest!
3. When communicating in touchy situations, first TELL the other party that you'd like to discuss something with them, then ASK if they are up for hearing what you have to say (if not then ask when), SHARE that you feel this is a sensitive issue and that it's important that you get to say what you feel without being interrupted and that you'd like to listen to what they have to say as well.
It's absolutely reasonable that you're feeling as hurt and abandoned as you are right now, and it's reasonable that the sister you helped a great deal be as giving to you as you were to her. She probably is simply clueless or hasn't reached a certain stage of maturity to be as considerate as you are. It might be she'd respond favorable to a nudge from you, or it might be she's just that way.
Either way, I never had much success with people reading my mind (and see countless relationships where people strangely hope others will read their minds) but have had success with communicating clearly. The HARDEST part is to not let my anger speak when I'm upset...but the greatest rewards have come when I remained firm in my conviction that I, not my emotions, are in charge of my actions and choose my words carefully. I have to stop and ask myself what is more important: that I vent at the person I'm frustrated with, or that a favorable outcome is achieved.
The BIGGEST message I want to give you is that I'm sending you a GREAT BIG HUG and support for your situation! Bedrest has got to be a great challenge, and you deserve to be treated like a princess while you grow your baby! I hope you resolve your conflicts and get to relax in the final weeks of baby-growing, that your birthing is a beautiful experience and that you're surrounded by loving family and friends.