I'm upset that my bil had a no kid rule at his wedding except for his niece.
Why was his niece invited to the wedding and not anyone else?
She is the only kid that tends to listen to adults.
It is understandable that you are upset that the rules don't seem to apply to everybody and you can share how you feel with the couple. It would be better if you told them how you feel so they can realize that it is unfair.
I wasn't upset about that, I was upset about what I did for him not being appreciated.
Weddings are really stressful, so I think it's best to keep frustration to a minimum and just enjoy the wedding. It's your BIL's after all, so it's their rules even if it's not fair. Maybe you'll come to enjoy the event in the end!