Following keepingquiet's post. Can you liaise with the other people cut out by your daughter, your SiL and his family.
This strikes me as a mental illness problem rather than a grandchildren problem and to deal with that you need to all work together. Your SiL, s KQ points ut, has a legal right to see his children and he could take your daughter to court to get access. If he cannot afford to, can other members of the family, including you make a con tribution to help him.
I think the problem you and her exhusband's family are facing is a joint problem and it should be looked at as a united group concerned with the mothers mental health and fitness to look after her children.