My mother-in-law does not have a good relationship with her other daughter-in-law. The daughter-in-law is very sensitive, young and slightly standoffish, and the mother-in-law is also very sensitive. They seem to have a barrier between them for one reason or another.
I know my mother-in-law is very sad that she doesn't have a good relationship with her other daughter-in-law - she confides in me - and it has jeopardized the relationship between her and her son. The son even told his mom that she was not welcome in their home.
I feel that the mom involved deserves some respect, but at the same time I think everyone needs to be responsible for their actions and words, even when they think they are not wrong.
Should someone intervene and tell the daughter-in-law that she should make amends with her mother-in-law and/or grow up?
Answer: Wrong question. It should be: Who gets to intervene to tell you that the mother-in-law is the one who needs to be told to grow up? (Answer: Happy to!)
The mother-in-law confides in you, so you already have her attention, respect and sympathy, as well as some standing to get involved. On these grounds alone, she's the one you need to talk to.
But more important, she's the longtime family member while the daughter-in-law is the newcomer. A good host - yes, host - will be indulgent of the newcomer, forgiving of missteps, and dedicated to the long view of bringing everyone into the fold.
Meanwhile, the daughter-in-law and son are a family unto themselves now. The mother-in-law's role is to respect that new family, and support the union in any way she can do so sincerely. When that's just not possible - when the newcomer really is a nasty piece of work - it is then her duty not to add to the challenges.
When a parent just doesn't like a child's choice of mate - or vice versa - there is only one moral course: Find as much as possible to like, and leave the rest alone.
The daughter-in-law has responsibilities, too, to make an effort to accommodate her husband's family. But if you have no history of confiding in each other, your intervention will come off as scolding and meddlesome - validation, in other words, for her decision to shut her husband's family out.
Your mother-in-law needs to be careful, too. If she withholds any overtures until fairness needs are satisfied, this will only deepen the estrangement. She owes her son and daughter-in-law an apology for her role in the estrangement. Period. The high road is wide open and waiting for someone to travel it here.