Thank you all for your helpful replies.
I actually sent the link to this thread to DP so he could see exactly how much it's affecting me in black and white.
When I got home we spoke about it albeit briefly, but what is apparent is the whole situation makes him angry as he doesn't know what to do.
I feel a bit in limbo as I'm waiting for him to show me that he will stand up for me.
But yet again he puts me in a position where he says nothing and I'm the one explaining to his dad of all people what the problem is.
And yet again, I get told to just ignore and move on.
Easier said than done.
I can see that my son won't have a relationship with his cousins, uncle or aunty, and I can see how my DP won't have a relationship with his brother or nephews and nieces.
But I've done all I can, I'm exhausted and stressed because of trying for them, when they can't even talk together.
What it's really illustrated to me very clearly is how a family can portray themselves as being a loving close family unit, and actually beyond the surface they aren't at all close and don't even talk to each other when there's a problem.
Leading to issues like these that just fester and grow until it breaks everything apart.
I never wanted to be a part of a family like that.
I don't want to fake it for family occasions (what I'm expected to do).
I don't want DP being so weak that he won't stand up for his future wife.
That he won't speak to his (younger) brother for fear of what will be said.
All I wanted was for him for the first time to say to people -
'Tiny has been treated very badly by SIL and continues to be, and I support her and will not accept any bad behaviour in the run up to the wedding'
I also expected him by now to have had a conversation long ago with his brother who is also his best man to say 'what's the issue? If there is, fine we then know and can move on. If not, leave us alone and don't bother tiny or my family any more'
By now he also needed to say to his parents 'you know that SIL has treated tiny badly and continues to. There is no excuse for her bad behaviour. Please don't exacerbate tinys anxiety and upset in the matter by telling us to just ignore it'
These would all be mildly acceptable to me, as in an ideal world, someone needed to really stand up to her by now and tell her to just stop.