Being told that a newborn *will* be at my wedding.

You aren't being unreasonable. It definitely is rude of them to expect this of you. Is there a compromise to make? Could someone not in the wedding (maybe one of SIL's family members?) watch the infant during the ceremony, and then during cocktail hour/reception go back to SIL? Then the newborn would not be in the room to disturb the ceremony, but you are also pleasing them by allowing the baby at the wedding at all. Also, as far as loud noises/unvaccinated/etc, that isn't on you. That is the parent's call.

Nope you're not being unreasonable at all. And I say that as new the mom of a newborn myself! She needs to sit this one out and stay home with the baby. Newborns are so unpredictable.

Yep you get to determine who is invited to your wedding. One thing to consider is that they may not make it to the event if they have a newborn.

I would stick to your original plan and keep to the no kids rule if you and your future husband want that for your wedding. Also, bringing a newborn to a crowded place could be risky with them getting sick or compromised. Maybe they can plan and find a babysitter.

A newborn should be the exception and in my family not allowing this would cause problems throughout the greater family. It is your right to decide you don't want them there. But be prepared to pay the price. At minimum I would expect that both Sister and Brother in law will not attend the wedding at all. At worse, lines will be drawn in the family and you may wind up with an irreparable rift.

I personally think (and in my family) you should just make an exception for a newborn and allow the parents to decide where they are comfortable being with their baby. If your wedding is 2 weeks from her due date, the baby could be even less than 2 weeks old. So it might all be a moot point. They may not come at all due to being overdue, or still in the hospital or because mom is feeling not her best due to just giving birth. To me, it just wouldn't be worth it to try to impose any restrictions on them besides: "if you come, please remove baby if they start to cry." But this is my comfort level and what is acceptable in my family. Only you know what is acceptable to you and your family (both bride/groom and the greater family dynamic)

That's tough. I would lean toward letting her have the baby there, because I wouldn't leave my newborn, but the part that makes me nervous is hurting the feelings of other people who had to arrange child care and leave their children.

"newborns are usually the exception to the rule," no. For me, it was a pretty big deal to NOT have have them at my wedding.

That is EXACTLY who the "no kids" rule was for, it was not for other children, it was babies. I did not want babies at my wedding.

That is your right to insist on this--but understand you are probably making a sacrifice that your relative will not be able to attend due to this rule. So if having a child at your wedding is a bigger deal to you than having that guest there, stick to your guns.

If you'd rather have that guest there, make an exception.

But you can't have it both ways.