My MIL has been a difficult person to get along with over the years. She has a very strong personality and is very opinionated. If you disagree with her, she will not like it and there will be a lot of tension until she calms down. She was a very authoritarian mother to her 5 children, being very strict, with a lot of wooden spoon hitting. My other half left home to join the navy at a young age and became independent from his family, and since then hasn't felt a strong connection to them. One day, many years ago, I suggested to him that he should think good and hard about seeing more of his family, especially his mum, as time is precious and the past is the past, etc. It did the trick and he started to be part of their lives again. He hasn't forgotten what happened in his childhood, but he still has his family and that can only he a good thing.
I think the point I'm making is, that it's not up to us daughters in law or sons in law to call the shots with in laws. We should only encourage good relationships, by putting our sensitivities aside and keep family on the same side. We don't have children, therefore there are no grandchildren in the equation. However, I didn't have grandparents when I was a child, and I really feel that I missed out on having extended family to turn to.
I sincerely hope that your DIL will come round and see what a wonderful grannie you are, and that your son will realise that his mum is suffering.