My daughter won’t let me see my grandson!

It sounds to me as if the son lives with her mother. They are leaving the UK soon and as OP has MS, they have to visit her to say goodbye, but can't, because of the daughter?

It also sounds to me like the daughter is a right madam rather controlling.....coming to visit, taking over and and ruling the roost. I'm afraid you need to put your foot down and show her who's boss.

I m also puzzled who is staying where? you have given up your bed for her but she doesn’t seem to be with you ?
She doesn’t get on with her brother, well is he living in your little flat too?
Your mother tells you he’s asking for you does that mean your grandchild is asking for you or your son? and are they now staying with your mother ?
Sorry I can’t offer more help but it’s a puzzling ask

I don't quite understand the time line here - one minute DD is staying in your flat with you, and then you had a falling out over her brother , and now she has gone to your mothers place ? Is that right ?

I have a son and daughter who don't get on , and I can only suggest that when you are with your daughter, don't talk about your son ( you already knew she didn't like him ). Why ask for trouble when she lives overseas and you hardly ever see her ?

It seems that she doesn't want to see you - if so you might have to hold back if you don't want to have more trouble . Try to mend some bridges and next time things might be better.

I agree that more details are needed here.

I thought she was staying with you? Where is she staying now?

What triggered this issue? What was it about your son that set her off?

Would it be possible to give more details about why your daughter has always had problems with her brother and what the fall out is about?

Take the bull by the horns and go to see them. Apologise if you have to. She'll be gone soon and you don't want to miss saying goodbye to him.