I had an absolute witch of a mother-in-law. She and my father-in-law tried to get my husband to divorce me when we had only been married three years and I was pregnant with my first child! Ever since, I have made it a rule never, ever, to interfere or get involved in my children's relationships. They know I am there all the time to support them and their partners if they need, but I will never, ever, criticise or judge. It's none of my business. This situation is deeply dysfunctional and sadly it seems there is no easy solution, but it is up to your son and daughter-in-law to work it out for themselves. I would counsel against resorting to solicitors unless desperate, as they only tend to make bad solutions worse by pitting one party against the other. I say this as a former (ie retired) Family Court magistrate. If the dispute did end up in court, it is most likely the mother would end up with custody of the little children and the father have regular access. What happens with the older children is irrelevant, as they are not the son's responsibility. I do have to say I wonder how someone can say they 'love' someone else when they feel they are treated so badly by them. That's not love.