So your mother rode roughshod over your brother & his then fiancées wedding plans?
Pressured them to not only invite children when they wanted a kids free wedding but to include them in the wedding party! How old is her niece & how old are the other children that your mother pushed on them? So similar ages - I can see why that would seem unfair, still NOT your mothers business to insist -that's never going to go down well - and not unusual for bride to only have attendants from her side of the family. It IS rude to have someone as an attendant not be welcome at the wedding breakfast!
"They’re people, not a table setting. Pricks" exactly! Shoddy behaviour.
Your mother was WAY out of order. ESPECIALLY the insistence of a distant cousins attendance.
"but naturally wanted her whole family there" but it's NOT HER WEDDING!
I personally disagree with child free weddings but I would never push children on people! I've never been invited to one, all the weddings I've been to have been (to my mind) traditional family affairs with guests of all ages BUT if I had been when dd was much younger I'd have simply declined. Personally I do think it's rude to have brides niece invited and NOT grooms niece & nephew especially as they're similar ages. But again in OP'S position on this, I'd have just declined. Bride & groom get to have wedding they want, they don't get to insist on attendance.
The treatment of GUESTS was appalling too!
To be in tears at all this is ridiculous drama queen behaviour (as was Drs & your husbands behaviour).
Honestly it sounds like your whole family are seriously lacking in manners! You all ought to be thoroughly embarrassed!
POINTLESS causing an argument now with your brother & sil - what's done is done. You ALL owe each other apologies, but particularly your mother as she caused all the tension. And what on EARTH would your dad have said??
"Bride and groom know nothing of upset but I want to talk about my husband being isolated without a drink and how rude his brother-in-law was." WHY? At this point what would it achieve? Are you actually likely to see him again?
At this point least said, sooner mended.
And maybe all sign yourselves up for etiquette lessons!