I very sorry you're in this unpredictable situation. Its sounds like your DIL controls what your son does - meaning if she's upset she thinks its okay to alienate you and that's wrong. Can you speak to your son privately and let him know, all you want is to see your grand daughter. Try to avoid placing blame but do say how you really miss the interaction and feel lost when this alienation occurs. As a last resort you could sue for grand parent visitation but this might cause more animosity and cost money. This is also damaging to their child - children get attached and don't understand why a loved one stops having contact. I wish you a peaceful resolution.