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

You are being manipulated. I would do things a little different. I would let my sister know...I have plans for every weekend next month except this one. If you want to go out and use my services it will have to be on this day. Know it will probably be an all night thing so plan a slumber party for you and your nieces and nephews. Make sure your sister knows ahead of time when you are going to able to babysit and stick to it. Limit it to once a month and stick to it. Tell her she will have to plan in advance and can't just call you on short notice. Your Mom should do the same. This will give her twice a month to go out. If she wants more she will have to make other arrangements. You let her know in advance when you were available. I am only saying this because your nieces and nephews should be able to spend time with you. If she was a friend I would tell you to tell her to stuff it. lol