Should I be forced to babysit my niece?

My brother and his wife had a daughter last year and my relationship with his wife has never been good, ever since she’s been married to my brother she has hated me and my 2 sisters. There was never a reason why she didn’t like us. After she had her daughter she changed and suddenly wanted my help with the baby, she started acting nice which she never did in the 5 years that they were married for before they had a baby. I love my niece so I didn’t have an issue with babysitting her but she would ask me to babysit multiple times a week, it was becoming a problem because I felt like I always had the baby and couldn’t do the things I wanted to while she could go out shopping and meet up with friends. The main issue is that she treated me like shit before they had a baby, she never bothered with me and never wanted to spend time with me, she just sees me as a personal babysitter. If she was nice to me from the beginning of the marriage then I wouldn’t really mind but the fact that she didn’t pay us any attention or care to bother with us is why I don’t feel comfortable doing her favours. I know if I had a baby she wouldn’t be taking care of mine, it’s like she feels entitled to just dump her child on me whenever she wants. She always puts me on the spot and asks me in a way where I can’t say no, she pressures me to look after my niece. Multiple times a week is too much in my opinion, I’m 22 and I do have a life of my own. I’ve decided to ignore any messages she sends me asking to babysit, I don’t know why people have babies and then expect other people to raise them. Am I being selfish?

Should I be forced to babysit my niece?
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