Well OP, if you really want to see who this guy is, do not involve yourself in any of the plans or catering.
"This is a surprise birthday weekend for me, I'm not involved in any organising of any part of it. Ask OH" on an endless loop.....
How he reacts to this will tell you a lot.
Also, I am really suspicious of the whole thing.
Men generally like it easy. You handed your birthday plans to him on a plate. Venue, parents organised to take the children. So easy.
So the fact he undoes all of this, re-arranges for his children to join, and your parents and sister and all the logistics involved is frankly unbelievable.
Yes, I have hugely generalised above regarding men, but I believe it to be true.
I also think most women in that situation would take the information given to them as a gift and go with it.
I think the change came about to facilitate his children and everything is a distraction and subterfuge.
There is no way this weekend is about you, if he has had 6 months to arrange it, and he hasn't done a thing about the details.💐