We went to a wedding yesterday and of course our daughter came with us. I know the relatives would have been terribly disappointed if she didn't. Unfortunately when it comes to entertaining her that responsibility falls on us, her parents, and it is hard to have a good time and visit with other guests while trying to calm a teething baby.
Not to say we don't enjoy having her with us. We do. But at a wedding it's hard to keep a baby from crying or yelling or fussing during quiet times such as the ceremony and toasts. Halfway through the ceremony Dave had to take her for a walk outside. During the reception I took her for a walk to the puppy store down the block so he could eat and then when he was done he met us there so I could eat.
I want to stress to you that if a guest doesn't want to bring her baby or toddler to your wedding - even if the child is your niece or nephew - say okay, and offer to help find a sitter if the family has to travel to your wedding location. They'll appreciate you not giving them an argument. And if the wedding is at a hotel the babysitter can bring the child to the reception for a short time if the parents and you want them to, and leave once the child starts getting fussy.