CrazyH, I'm sorry this happened and hope your GS wasn't too upset by it. Also I know it hurts to see loved ones fighting, especially AC. So my sympathies are with you.
I'm a little confused, though, as to whether there was alcohol at the party or if you drank (a little too much) afterwards b/c you were upset. If it's the former, I'm going to chime in w/ those who say there shouldn't be alcohol at a child's birthday party and that might be why DS, DD, and DIL got so out-of-control.
I'm also confused about who argues with whom. You say, at first, it was DS and DD. But then you tell us DIL "ended up crying" and that DS was simply caught in the middle between her and DD. I get that you were a little drunk when you were writing that, LOL, but now can you clarify?
IMO, all 3 were very selfish to let that happen at at child's birthday party (unless they were drunk which was also a mistake, IMO). Unless the bday boy was their child, I think DS and DIL should have left as soon as they saw things were getting unpleasant. Same for DD. Better yet, somebody should have changed the subject before things got out of hand. It's awful to have these arguments blow up at family events, but especially at a child's party. If I were the parent of that child, I'm sorry, but the offenders would never be invited to my child's events again even if they were my own brother and sister. Or I just wouldn't have family to my child's parties, only friends.
As for your "big bday," I know it must mean a lot to you. But all the more reason why I wouldn't take the family to London. Why put effort and money into an event that might very likely be spoiled by selfishness and arguing? In fact, IMO, they should be planning a big day for you, but chances are, the issues between ODS/DIL and DD make such planning difficult. You might be happier going out w/ friends or celebrating w/ each of your AC and families separately. If your AC ask why, tell them.
Wishing you the best, in advance!