She is in a wacky place, that's for sure. I'm not a vet, but there is something that is antagonistic in your communication with her. You are not treating her like a friendly co-worker. You are not treating her like a neighbor.

When my H and I communicate about our D's schedule, I almost always start off with, "Hi H."

I think you are pushing her buttons. What might happen if you said, 'S11 and I are having a great time today, and have plans this afternoon. We agreed that you'd pick up S11 at 4 pm, and I'd like to stick to that. I am busy so I can't reply more at the moment."

Then, after she picks him up, you could craft an email to address the request that you take S11 on her nights. You can reply politely, without sounding obnoxious or apologetic.

What I've tried with my H is validating, but holding firm to my needs. Something like, 'I can understand that it is tough to pack up the house and entertain S11. And I need my space and time, too. I'm sorry I can't accomodate you."

The angry, curt texts in the moment always seem to escalate things. I'm sure it's scary for your S11 to be stuck in the middle-- mom asking to talk to dad; dad refusing; S11 having to tell mom that dad refuses to talk to her. That is a terrible position to put him in, don't you think?

Take some deep breaths, find your focus. You can do this.

Me 38 H 40
D 3
T 8 M 6
BD 10/2013