Thank you so much everyone, such a lot to think about here.
I have had the call, it was ok, DM did her usual, 'gabble, gabble, gabble, don't take a breath and be bright and happy'; the usual let's not confront this.
So I did, as I usually do because I find avoiding difficult conversations just builds resentment.
I said that I was really upset that I hadn't been welcome at theirs especially as it was my birthday. I said that my sadness impacted on the rest of the day, which we ended up cancelling.
She sort of detached herself from it - 'oh I didn't know what your DF's message said - but that it wasn't about my DB, that she would have let us be there but we couldn't have used the toilet'.
I asked why she hadn't phoned, rather than send a message, that we could have talked it through, that I could have assured her that we would have made sure we'd gone to the toilet elsewhere.
She said she didn't know why.
The conversation didn't really go anywhere, just that's how it was, we couldn't be there.
She did hit back with 'oh why did we move here, no wonder we stayed abroad so long, we should have just stayed'.
However, I moved the conversation on to the future and how would we be able to visit without there being this sort of issue, adding that it was unacceptable that my DS was also made to feel unwelcome at their house.
I asked her if she wanted us to make an appointment to see them so that they always know when we are coming - she said not.
I made it clear that in that case, if I call in unannounced at their house and DB is there, that I'm fine with and won't be sitting outside until he's gone, but if he doesn't want to be there with me, he can of course leave.
I suggested that she and my dad have a clear conversation with my DB too so that everyone knows where they stand if this occurs. She says they will talk to him too, but that when they've tried in the past he shuts down. I don't think they will talk to him, but I will ask next week and ask what his response was.
I said that if this was a situation in my own family, the least I would expect of my DC's is good manners and respect whilst they are in my house. That my DB is rude, there was no need at all for him to ignore my DS and also discussed other family events when similar has happened, how extended family members have expressed how ignorant my DB behaves.
I expected a call from my dad, usually if I challenge their acceptance of my DB's behaviour, my dad rings really cross, usually 'you've upset you mum'. He hasn't and I wouldn't have answered anyway.
We'll see in the future what happens. I'm going to keep talking to them about it and following through with what we've agreed so far.
Anyway, I ended the call with a bit of a low blow back. 'We're off to my MILs tomorrow, she's invited us to celebrate DH's birthday'.