Two of my DGC live abroad. I visit as often as I can and as a result have seen parts of the world I might otherwise have missed. Young people today travel and sometimes settle abroad much more than our generation did but they often miss home and return to their roots, so don’t give up hope. But mending bridges with your DIL before they leave is vital, talk to her about how you feel, she too is now a mother and may well understand your feelings of loss.
There is always Skype to bridge the gap and remember there are lots of us in your situation, you are not alone and there is a lot of good advice from your fellow GNetters to help you come to terms with and make the most of being a long distance GM.