It’s hard to judge this as an outsider op.
Does your brother have form for being this reactive and aggressive normally?
I think he was very much in the wrong to react with such immediate venom to quite a benign remark on your part, in fact it could be argued you were showing very normal interest in your niece and asking a fairly standard question, but he obviously took it as a criticism of his parenting, and that’s a touchy subject for some.
I doubt it was because you ruined a surprise party. People’s reactions to arguments are almost invariably more about them than about you.
If there was any surprise to be ruined, he did that by answering in an overly agressive manner instead of just replying with a mild quip.
I suspect he has recently been criticised by his wife or daughter for not being involved enough in family matters or something like that and he was feeling guilty? Or there had been some previous dispute about the party such as the cost or timing of it? Is that possible?
Or it could be that teasing between siblings is one thing but he didn’t like you involving his dd in it and he wanted to draw a clear line there?
Or as others have said, this wasn’t about the party, and you had simply teased him one time too many on Sunday?
The laughing at you getting tearful sounds horrible though and a bit bullying.
Whatever the rights and wrongs of it though, your brother was absolutely wrong to be so openly agressive and hostile, especially on a family walk, and especially in front of dc. If he had a problem with your teasing, he should have taken you aside privately and addressed it calmly.
I have a somewhat bullying brother so I understand your shock when you discover that what you think is a safe relationship, really isn’t.