Sister's Baby & Etiquette

I think if you allow all your nephews, you need to give the option to invite the niece too. No one likes to feel their kid is being singled out or that others are getting special privileges. You should talk to your sister first, though. It's possible she already plans to get a babysitter and this won't be an issue.

This is unfair to your sister and niece. You should to apply the rule to all children or none. 8 year olds can be just as disruptive. If your sister does not have or regularly use a sitter, chances are she will not go out of her way to find one. If you find one for her, she is within her rights as a parent to decline. Even if you find a family member to skip the ceremony, it's not fair to them or your niece. You can sit her partner (If in the picture) or a trusted family member or your sisters choosing at the end of a row so they can dip out if she starts to get fussy.

Usually it's best to invite in "circles" of level of relation, so if you're inviting the nephews, then your sister might be hurt if you don't invite her child.

I agree with Jacks that you'll likely hurt your sister by inviting your other nephews and not her child.

FWIW - we had 5 children under the age of 3 at our wedding and no disturbances at all.

I do agree that there could be some hard feelings, but I also think that infants/toddlers are different from children who are a little more grown up. A baby or toddler is more of a wildcard because you can't tell them "Hey, no crying for the next 20 minutes" - if it happens, it happens. With children who are a little older like your nephews, they have some understanding of the environment and can comprehend that they need to be on their best behavior. I personally would talk to your sister and let her know that you're worried about your niece possibly getting fussy and ask if she has any plans to A) make sure that she can enjoy herself that night celebrating with you and B) alleviate some of your concerns of potential distractions.

You could also offer to pay for the sitter. Since there are still 4 months before your wedding, if your sister doesn't have a regular sitter she uses, there is time to find one and you could potentially help her in the search so that she doesn't feel offense over the idea or that you're creating a new task for her.