I think you may well be right that your instincts are telling you that DIL is a strong, assertive character and rather bossy. I understand that it’s then very easy for a mum’s thoughts to run on further to thinking she is fully controlling, coercive and isolating her son from his family. You may be right, so make it so that he can turn to you if necessary,
And I want to help but I don’t know how to help.
Let me just give a few thoughts which might help.
Remember that you have no evidence that she is corercive. You don’t know their relationship very well. Your heart doesn’t want to see them as a couple because you sense something is off. Even so, not thinking of them as a couple doesn’t help your son. He has said as much so believe him. Trust him. Trying to hold on to him as if he’s a single man doesn’t help his marriage. It alienates DIL. And demeans him. Makes things worse for him.
DIL has spidey senses too, just as many as you have. And she may well also let her thoughts run away with her and exaggerate the fact that you don’t like her and won’t accept her as your son’s wife. So now, the hill you have to climb to get to know them as a couple, is even steeper.
Send her the grey mug OP! Every time you buy him a shirt, buy her something too. See something you think she might like and send it to her without sending him anything. Act towards her as if you fully accept the marriage and as if you like her, appreciate her and value her. As if she were your very own daughter and not the enemy. That’s what he wants. It may well make his life easier ….and that’s what you want.
Your son will always be your precious baby and you will never forsake him or betray him. That’s normal. He also knows that. It’s hard to get to grips with the fact that they are in their 40’s and no longer in need of your protection. Remember that he’s always been independent. Remember that he chooses to stay with her. Remember that he has asked you to treat them as a couple. He hasn’t asked you for help so you can’t rescue him. But you can quietly understand the situation he is in, whatever it is, and go along with what he says. Maybe that’s the way he wants you to help him for now. Do it so that he knows he can rely on you for the future. He may want to end his marriage further down the line, but for now there is no reason to think that he is asking anything of you except for you to understand, send her gifts too and value her. Maybe she complains to him about you not treating her like a DIL. So you can help him by giving her nothing to complain about in that respect
Try to find something about her that you like. Even just a little bit. You have to grow to love her. Force yourself to love her. Your son loves her OP. He’s chosen to stay with her so make it easier for him, not harder. Say to yourself….I love her for her sense of style/her ability to cook nice food for DS/her outgoing nature/her wish for me to send her gifts too. Find one tiny little thing you can love her for and build on that. When you send her a gift, put a card in it and write something nice for her to receive .eg I saw this, thought of you and think the colour will suit you, really hope you like it. xx
Be thoughtful about her no matter how hard.
Ask them if there is a time you can face-time once a week. It helps to see facial expressions. See if they are open to that. They have very busy lives. Let him call you. You feel frantic inside about him, but just reduce the number of calls. Remember, he can always call you if he needs help. Trust that he knows what he’s doing. Keep the calls light hearted.
Never criticise her in front of him, or in front of others. Just tell him you love him, that you understand that life isn’t easy with his work schedule and that you are always there for him and your DIL too. Reassure him that things will get better in time.
Yes, of course encourage your other son to visit. Give him gifts to take for each of them. Don’t make it a rescue mission though. Just an opportunity for them to re-connect and have a lovely time. Remember that if your son does confide in his brother about something , you may not be told anyway, so don’t probe.
Finally remember that sons never want to tell their mums the bad stuff. They want to protect them from it. They hate hurting their mums. Be respectful of that. He hasn’t given up on his marriage yet, so you shouldn’t either. Trust him to do the right thing.