Am I being a selfish jerk or is my sister taking advantage of me?

Anonymous
My sister works every night, no days off. She expects me to baby sit every day between 7pm - until she gets back from work, which is between 3 AM to 6 AM. The problem is I'm trying to focus on my own priorities. I love my nephews and nieces to death but I've been there for them, taking care of them and babysitting for my sister since I was 12 years old (I'm now 26 going on 27 later this year) and I just don't want to baby sit anymore. I need to focus on finally succeeding my goals but babysitting everyday and getting home late or very early in the AM is slowing me down. I'm trying to focus on joining law enforcement. But if I tell her I can't babysit anymore she'll guilt trip me. I want to help my sister because she and her husband work very hard but I feel like I can't bend over backwards for her anymore. Two of her kids are teens now, and the other two are nearing their teen hoods. Am I wrong for thinking the two teens can watch over the two younger kids? The oldest is 15 and the other is 14. I get that she might be a little worried but she can set up an alarm system right? Am I wrong for feeling like I need to start focusing on my own priorities and that it's way past due? I'm nearing my 30's and still haven't accomplished anything. I get no free time either because if I tell her I'm going out for a movie or a meal, she'll get upset and tell me that I'm supposed to be there babysitting. It makes me very upset when she does this but I suck it up because I don't want to have any conflicts with my sister or anyone in my family (I guess I'm passive aggressive), which is also why I always tell her "yes" when she asks me for a favor. If I'm wrong, correct me but if I'm right, I would like some advice (beside growing a set)
Updates
+1 y
I was able to talk to my sister last night and although she looked very disappointed she understood and to my surprise took it better than expected. Thanks again everyone for all the help and support.
Am I being a selfish jerk or is my sister taking advantage of me?
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