What to do with MOH a week before wedding? (long - update)

This week has been hectic for me, to say the least. I'm getting married next weekend and trying to get so many things done beforehand. This past Wednesday, my sister (the MOH) calls to tell me that her friends who were supposed to babysit my 2-year old niece said that they were uncomfortable with the idea and backed out because my sister and BIL would be two hours away. Back in November, we told my sister that my niece is invited to the RD, after wedding party, but not the wedding, and she agreed. Two cousins of ours had a friend each who were willing to babysit and my aunt and uncle (they're not married to each other - different side of the family) said that they trust these friends to babysit in their (aunt and uncle) own home. These potential babysitters are 22 and 23 years old. I offered to pay the babysitters money, so my sister did not have to carry the financial burden. My sister told me a couple of months ago that her friend would watch my niece for the whole weekend. I thought the situation was settled.

As I mentioned before, my sister called me Wednesday to tell me her friend backed out. Then she said that she and her husband are uncomfortable with having strangers (cousins' friends) watch my niece. I said, 'So does that mean you're bringing Haylee (niece) to the wedding?' and she responded, 'Well, otherwise we can't go.' I then explained that I needed to talk with FI as I cannot make this decision by myself. I also asked her to understand that we told other friends of ours that their children are not invited as we are having an adult reception.

I am so numb at this point. I told FI and he was so angry. He does not want children at our wedding. We are paying a lot of money to have the wedding we want and we want no children. I'm also very angry because I feel that my sister gave me an ultimatum - either my niece is going to be at the wedding or they can't come. (they are traveling from North Carolina). One BM observed at my shower that my sister seemed to be insanely jealous. She was supposed to have a wedding last July but had to get married earlier due to Sept. 11. She is my younger sister and has felt this need to be the first to do things: be the first to get married, have the first grandchild, which she did accomplish. I personally don't care for this competition and try not to get involved. However, I am able to have a wedding (of which we are paying for most of it) and go on a nice HM and since she was not able to do this, I think (and my BM agrees) that she is trying to get some sort of attention or something. I also think she feels that she is not living the 'ideal' life (my sister is very into the perfect life where you get married, having 2 kids, a house with a white picket fence, and a dog). She got pregnant one month into a relationship, had the baby, married the baby's father 12 days later in city hall by the Justice of the Peace, and is raising her daughter as well as her husband's three kids, two of whom do not live with them. Money's a bit tight, as it would be for most people who have children, but instead of embracing what she has, she makes comments about my brother and I, about how nice it must be to order Chinese food for lunch, or how mice it must be to go out drinking every Friday night. She gladly accepted to be the MOH when I asked her in the fall, but now she's saying that unless her daughter can go to the wedding, she may not be able to come.

What advice can you give me? I really don't want to create a rift with my sister, but allowing my niece to attend the wedding would make my FI so angry (and I have to admit I may be a bit resentful as well). My parents are pretty ****ed with her, but my mother can't help with the situation because her mother is dying and she's dealing with that. I appreciate you reading this very long post because I don't know what to do. I posted last weekend how I can't stop crying because I felt so overwhlemed and I actually did stop and felt better until Wednesday when my sister called.