I would try to reconcile but, if this doesn't seem possible, you could seek a court order to be able to see your grandson. It's probably the nuclear option, as it will not improve the position with your daughter and SIL and may well make it worse. However, given the high level of contact with your DGC, a court would probably order contact - although this mat well provide further impetus for them to leave, if they are not already thinking of doing so.
My sympathies Debcz, it sounds like an awful situation. I wish you well and have everything crossed for a favourable outcome, although this may take time.