Oh dear, I am so very sorry. Let me say immediately: you write like a sensible woman. So let’s see what may be happening.

As you point out, adults are allowed to date who they please. When a sibling, relative or friend dates a person we don’t like or disapprove of, we may say so once and then we respect their choice.

Sure, we may not hang around as much, but overall, we try to keep the relationship with the person we love and quietly avoid the partner we don’t.

Your daughter-in-law has no respect for others. She punishes those who don’t do exactly as she dictates. That is abusive.

In healthy relationships, adults have the manners and etiquette to socialise with people they are not particularly fond of. It’s a normal part of adulting.

Why doesn’t your son stand up for his sister? It may be that he is abusive too. Or he may be frightened of his wife. Abusers have a way of harassing and scaring their partners and kids, bullying them until they do exactly as they are told. So what is going on with him is unclear, except that he is a grown man who is OK with bullying his sister. That is not acceptable.

What can you do? You support your daughter. She has done nothing wrong and she is being targeted, smeared, harassed and isolated by her brother and his wife. Do the right thing and stand up for her.

Your daughter-in-law will scream and make threats. Your son will support her.

Can you fix this? Sadly, that is tricky. Abusive people have no morals. They don’t care who they hurt; all they want is power. This means that nice people like you are fighting an uneven battle. Sense does not work.

If you can get the family together and agree to stand firm, reminding this couple of the rules of common courtesy, you may get them to knock off the bullying.

However, be prepared that your daughter-in-law and son will punish you by telling your granddaughter that she may no longer talk to you.

It comes down to a choice. Me, I would do what is right and support your lovely daughter and her fiancé. If your son and daughter-in-law want to absent themselves, then I would comfort myself with these thoughts.

First, with abusive people, the drama never ends. You walk around on eggshells. Living without all that would make your lives a lot better.

Second, your son is stuck for now. He is identifying with the abuser. That may not last. Your daughter-in-law may go too far, allowing him to see her clearly. Or he may at some point get away and reassess what he got into.

Third, abusers control their family for a while but often lose them when the kids grow up. All that bullying and drama rankles.

So not only may your son come out of this, but when your granddaughter becomes a teen, she will wonder what you are like and will likely look for you.

So, should there be a blowup, please know that it may not be forever.

What you do next is a choice. Given this is a tricky and emotional issue, I think you would profit from having two or three sessions with a professional skilled in domestic violence and abuse. Talk through all the options.

Whatever you do, make sure you comfort your daughter right now. She is the target of awful bullies and she needs her mum.

Good luck, and know I’m thinking of you.