Freaking out

I am freaking out. I just spoke to my sister, and basically he is starting a new school tomorrow, and was going to be collected by a nursery in the town where she lives, as the owner's child also goes there, and then do the after school session at the nursery and then my sister would pick him up. That has all fallen through, and she now doesn't know what she is going to do, as she works, and the school bus doesn't go to their town as he is going to school out of area. So she has asked me if I could look after him 3 afternoons a week after he finishes school. I really, really do not want to. I love my nephew, but I find children really stressful, and after about 5 minutes with them I am just this anxious, irritable mess. There isn't even one small part of me that wants to do this. 2 days a week there is an alternative, as there is a childminder who could have him, but basically the third afternoon she doesn't have an alternative, and so I am basically being forced into it, and she would rather I had him the other 2 afternoons as well. I don't know what to do. I am really stressed and freaking out. I tried to think of an excuse but I couldn't think of a single legitimate excuse. I hate being put in this position. If it was just a suggestion then that would be fine, but I don't feel like I am being given a choice, and that pisses me off. I don't want to look after him. I know I should want to, because he is my nephew, but I don't. I do not want to. At all. Not any afternoons a week. If it was just a one off now and then it would be ok, as I could do it if I was feeling up to it, but I really do not want a bloody weekly committment of childminding. And I don't know what I can do. This is shit.