Tess - well done, you really are amazing.
Hats off to you for how you're handling this.
You sound spent, unsurprisingly after the few days you've had.
I hope you manage to relax a little bit when you're away and clear your head.
And I really hope that your DD and DH manage to sort something sensible out without you.
Stinky
"can you really expect DH not to back her up"
He shouldn't be "backing her up" in a way that sets her against her SM, he should be supporting her properly as her father.
It's totally and utterly unfair to tell someone to "stop fighting" when the only reason they're fighting is to stick up for themselves when they are being badly treated.
Given that both H and DSD are expecting Tess to carry the entire burden here it absolutely is about her.
It will stop being about her when her husband takes on his share of responsibility for this situation and starts working through with his daughter how this is going to be handled.
I actually can't believe you think Tess should just shut up and put up in this situation.