I really feel for you. Try to remain friendly with both sides, despite the acrimony, never take sides. You could try sending DiL the occasional text saying that this must be such a difficult time for her, but that if there is any way you can help, to let you know. When things settle down she may be glad of the odd babysitting session as she is on her own.
Perhaps you son might be able to briing the children over on one of his access visits. You might be able to get letters or books to them that way. I'm afraid all you can do now is bide your time and be there for the whole family, not easy I know. 