How do I get my sister in law to baby sit?

My sister in law hasn't refused to babysit lately, she's just been avoiding me. She was willing up until a few months ago, but then around Christmas she dropped me like a hot potato. That coincided with the announcement of my third pregnancy. It was completely unplanned and I was really counting on her to help me. I work mostly full-time, but my brother takes care of her so she doesn't work at all, besides cleaning her little house. Every time I call her she's out. I resorted to calling my brother and asking him to ask her. He just told me to lay off!? He says she's not feeling well. Her grandparents are ill, but they're really really old and she used them as an excuse to duck out of all three of my baby showers. I think she might be upset that I didn't let her throw me one but her house is too small to hold even one third of the guests that were coming. My husbands side of the family is huge and they never miss an excuse to get together. She seemed to understand. But then she ditched my celebrations, including my twins sixth birthday. Then she sent gifts that were not on the registry to both my twins birthday and to my baby showers. Agh, it's like she's trying to get under my skin. Now she has gotten rid of her answering machine so I can't leave messages. I tried just dropping the kids off, but she wasn't home, meaning she wouldnt answer her door. I just heard that she's going back to school, but that will only be two days a week. How do I make her realize that she has obligations by marrying into my family? Two days a week at school means she could still watch the twins in the afternoons and the baby all day on the days she is off. I'm so put off by her attitude. This is her family and she needs to help! She has all day to do whatever she wants but won't help me at all. I've been forced to leave them with my other sister in law a couple of times and she's just not as responsible and the kids don't get their homework done with her, she also let's them eat sugared foods. I don't have the option of not working, my husband still has major student loan debt and a free babysitter was the only way we were getting by. HELP.