Selfish?

You tell your friends and family that you would do anything to help them out, just to let you know, then it happens, they ask you to do the one thing that you don’t want to do. Everything inside you says no, anything but that, anything…cooking, cleaning, grocery shopping, etc. However, is that being a good friend? As a teacher, I spend a lot of my weekends and summer avoiding my former students (I am not apologizing for it either, we all need breaks from our jobs), I don’t go to Target near the school, stay away from the mall, etc. Now, my sister whose husband has cancer has asked me to take her place and do Bible school at her church. First of all, I am a teacher for a living; therefore, it would be like going back to work, on multiple levels. Second of all, it’s not even for any of my nieces classes; I would be with a bunch of kids I don’t even know. I am honestly having a very hard time with this and she wants an answer ASAP. Is it selfish or wrong to say “no”? I am very conflicted about this.