STBXW is asking again about going in vacation. Not that it matters now, but should I ask how long the EA has been going on?
TT180, what's *your* purpose? It's hard to imagine a good reason for asking her this.
Is her being about to sleep with him making you rethink leaving? You could still make a stronger shift towards, "I don't want a D, but I won't stand in your way!", assessing and 180'ing your problem behaviors, and validating her frustrations. You have been writing for years about the toxicity of this relationship--it's also fine to let go.
What would be self-defeating would be trying to stop her.
I *would* reconsider your view that it's harmful for D2 to do overnights with you in light of this upcoming trip. I'm going to assume she's not bringing D2 with her and leaving her overnight care to you or her parents.