TBH before I had DD I would have said YANBU. None of my other children would have done this - a look and a very sharp "get your arse in their right now and do what you're told" would have been enough.
But DD is a different kettle of fish. Her tantrums have always been of epic proportions. And if she's tired and over-excited they are even worse. And the only thing that works, and believe me I have tried everything, is letting her huff it out. Trying to engage with her, trying to cajole her, or indeed order her to do as she is told, makes it worse. At 5 or so it would have resulted in a complete meltdown, screaming, kicking, yelling, and would have achieved nothing. Not a thing.
It was exceedingly embarrassing, very difficult to deal with, and really led to a number of family rows when everyone else was telling us how to deal with her, what to do, to force her, smack her, or whatever, and how no one else's children ever did this and all that she needed was some proper discipline.
You have to accept that your brother knows his child best, and although your mother is disappointed, there will be other years for photographs.