Never mind having to walk on eggshells, I would walk on burning coal for my Dgc.
I would go for the four days, be as helpful as you can without being intrusive and I would treat Dil normally i.e. as if nothing was wrong.
So, I would arrive, after greeting excited Dgc, I would give Ddil a big hug, lovely to see you, thanks for having us, you/the house looks lovely.
Then I would carry on the whole time as if I hadn't noticed she wasn't speaking and include her a lot, for instance when talking to Dgc say something like, I wonder if mummy would like that or let's see what mummy says.
You get me? Keep mentioning her, while talking to the Gc, basically be extremely kind to her.
I know that you probably want to strangle her but really try to build bridges. She's obviously hurt by some real or imagined slight from either you or your Dh, so be the bigger person.
Good Luck!
Times article claim that Waspi women are tone deaf and should read the room
