It must be hard to watch your child go through that, grannyhen, and I'm sure as mothers, we know what it's like to want to help and protect our children. However, she is also grown up with a husband, who know should have that role of protecting and caring for her, along with you. I don't know what it's like to have grown up children, but I watch how my Mum handles me/ my dh, and this would be my advice:
Offer support, be ready to chip when asked, don't worry if things aren't 'perfect' eg. the house is a mess, the oven isn't cleaned, the washing's piled up a bit. If they need you there to babysit while they go to RELATE maybe you could do it one of the days your dd doesn't go to work, perhaps every 2 or 3 weeks, rather than every week, or 1 long weekend a month.
Also, let HIM be her rock and support. Maybe he isn't doing a great job of it and you are tempted to provide that for your dd, but I'm sure with the help from RELATE, he will get there and she will love him for it, and it will be the making of their marriage.
As an aside, t sounds like your relationship with him isn't v good if you stay in your room when he's there. It just doesn't sound relaxed IYSWIM. So I can imagine that he doesn't really want that extra stress in the house.