How to Avoid Infants at Reception

Unless someone else is paying for your wedding, you shouldn't be forced into inviting people if you don't want them there. If you don't want kids at your wedding, don't invite them. It's that simple.

The only way to avoid hurt feelings and confusion is to go all or nothing.

Yeah with all the people you have listed, it is all or nothing. There is no way you are going to get out of inviting your future niece unless you exclude all children. Honestly, the baby will likely be nursing still at that age so IMO its not kosher to exclude them anyway.

I would just do no children under 18 to avoid issues.

I am not cherry picking which ages but I am drawing the line with the relationships. All 1st cousins are invited which include a few children between 7-10 but some of my older cousins have toddlers that will not be invited.

ETA: I agree with PP that a nursing child should be included though.. especially your niece.

Don't accept the money and pay for your wedding yourself. Then you can invite or not invite whoever you want.

When I was in college I was hired to do childcare for a wedding. Myself and one other person worked together on-site (but in a separate room) to care for the kids through the ceremony, dinner, and into the dancing. I reunited them with their families for about the last hour so the extended family could mush over the little ones before the parents took them home for bed. It worked really well. Parents were comfortable knowing kids were nearby, but they could enjoy themselves (no stress, no baby tears) for the evening. Family still got to visit together with them at the end. Maybe providing an option like that could help?

You absolutely cannot do this. Splitting up families is the epitome of rude. What would that invitation even look like? Mr and Mrs Jones and Ted Jones BUT NOT Nellie Jones. It will automatically de-class this high end affair you claim to be throwing. Just stick to your guns about no kids and accept that that comes with a loss of contributed money, or accept that there will be children at your wedding and this is going to affect your vision. If you were paying for your wedding yourself you wouldn't have these problems, but money rarely comes without stipulations.

Also please reconsider the 3 month old. Firstly, this is your FH sibling, meaning your new niece or nephew. Not a great place to burn bridges. It's considered pretty petty in most circles to eliminate nursing babies from any event, and at three months I guarantee you won't even know it's there. Honestly this comes off a little as you not wanting to share the limelight on your wedding day because your SIL mentioned showing the baby off at the reception. Not a good look. If you really decide to take a hard line on this be prepared for one or both of these parents not to come. It is not easy, feasible or emotionally simple to leave a baby that young behind with anyone. This may result in your fiance not having his own sibling at his wedding. Again, bridge burning.

If you don't give a crying crap about the reception why do you care if there are children there or not?