Such a pushover

Why can't I just say no to people? My dad just called. My younger sister is going to a friend's house that lives near me. My dad doesn't feel like going to pick her up again, so he asked if she could sleep here. I hesitantly said yes, but I was wishing I could say no. It normally wouldn't bother me to have her stay here. However, I haven't had a second to myself today. Neither kid took a good nap, they were asleep at different times, and I was really looking forward to having a few hours to myself. Especially since J didn't fall asleep until 1:00 this morning and I was battling with him for three hours to get him to bed last night. So, I just collapsed into bed once he was finally asleep. I haven't had any me time for two days and I need it so badly. I wish I had the balls to say no, but I am too scared of my dad.