Why do I feel so guilty when I say no to her? :\?

Actually, there are two elements at work here. First, you are finding it hard to say no simply because you are the caring type who not only wants to help, but who doesn't want to let them down as well. Secondly, because you are that type of person, (and i'm sure your sister knows this), she is playing the ole "guilt trip" card on you and even uses her own kids as pawns in the equation. The answer to this problem is not as difficult as it may seem. In fact, the solution is quite simple... your sister needs an attitude adjustment and a reality check all at the same time. She needs you more than you need her. Although missing the kids would obviously be difficult for any given time, try saying no to her for awhile and don't worry about being deleted from Facebook..tell her that actually sounds like a great idea..and then play the waiting game with her... she will be calling you and apologizing. And if she messes up again, simply repeat the previous steps and I can assure you, at some point...she WILL get it..