Cut the verbosity, the "message sending", and the sullenness:
W: So you still doing your <volunteer group> weekend the weekend I'm moving? Me: Nope. W: Oh? So you're going to be around. Me: No, S and I are having a big fun weekend out of town, not sure where yet, I'll be sure to let you know, but we'll be out of town from Friday to Sunay. W: Oh. So you won't be around to help then? (said with a negative tone) Me: Right. W: So it's just me and <her step-dad>? Me: That will be a lot of work! W: Well, I don't have anyone to help. It's not something you ask friends to help you with. Fine. (turns, huffs, and goes downstairs).
As for the childcare stuff, I'd strongly urge you to do it in your house. You do NOT need to be in her space, worrying about respecting her stuff, her rules, checking for evidence of her personal life, etc... Parent in your own home.
