Thanks to all. Hetty58 = yes, they have been struggling for some time and we have been babysitting while they attend couples counselling. We ask nothing about what went on during sessions because it's not our business to ask; we just try to support and encourage.
DGD's obsessive behaviour could be normal, that is true, but her parents are driven batty with it and react with irritation because it means she's often late for school because choosing pants and socks, for instance, takes so long. DD feels guilty about feeling like this, but it's understandable.
I think I view the difficulties as a problem because of the practicalities involved - they don't have a lot of money; DD works from home but doesn't earn enough to pay rent on a property. Arguably that's a while off now and anticipating how she'll cope financially is the least of it.
We are both trying to be as upbeat as we can and avoid displaying anxiety and distress round them. We want to support them all. DD's partner is lovely and a sweet man but he has his own problems and issues which have contributed to their problems. That said, I don't anticipate a bitter separation, should it happen - their big concern is their little daughter.
I realise that I do need to step back a bit more and let them sort themselves out (no choice really and there is nothing I could possibly do to fix anything). It's another hurdle to negotiate, even so, but it seems to happen so frequently these days that it's almost the norm, unfortunately.