Well this is my first time posting here soo.. don't hate me haha. Buut I'm gonna respond to this in a way that you prooobably wont like..
First of all, I don't really see what the big deal about the babysitting is, your dad probably just wanted you to have some happy bonding time with his girlfriend. And it sounds to me like she was kind of joking around when she made that comment about the little girls not making dinner. Yelling at her prooobably didn't help things.. It sounded like she was suggesting she would cook, which would be a nice thing to do, -not- a bad thing. And whether or not she said she was going to be your new mom, from your post it sounds like she is. Whats so bad about her at least trying to be a good mom? At least she sounds like she cares and shes not being a jerk or something. From your post it sounds like it could be a LOT worse and that your just over reacting, but that's just my opinion from what you've said. I don't really know what's going on as well as you do, so for all I know I'm completely wrong. But I think you should just give her a chance, and think about how much worse it could be. If your real mom's gone then your dad's gonna get lonely and want someone to live his life with, I say be happy that it's someone who's trying to be a mom.
Good luck.