Difficult question about kids in BP (LONG)

So here's my dilemma. I have only one nephew that will be 8 at our wedding and one niece (his sister) that will be four and a half. I really wanted them in the wedding and thought it would be perfect to have them as my flower girl and ring bearer and they can walk up together. I am not having any kids at the reception - my neice and nephew will just be at the ceremony and around for some pics.

Here's my problem. My fi's got a niece and nephew as well. His nephew is two and a half and his niece is only 10 months old. When we got engaged last year we felt his nephew would be too young anyways to be in the wedding and then who would watch him afterwards? (Our wedding is all the way in the Hamptons). Well, now that its only 3 months away, I feel like his nephew seems alot more capable of being in the wedding but I think its too late to ask his parents about it and I think I might offend his brother and sister at this point no matter what I do. My fi is kind of on the fence. But I don't want his whole side talking about why I didn't include the nephew.

What should I do????

P.S. Hopefully that made sense.

Maybe you can approach FH's bro/sis and tell them just what you said, that he seems more mature for his age than you originally though when you were thinking about how to put kids in the wedding, and understand that it's last minute, but you would be honored to have (the boy) in your wedding. The 10month old is way too small IMO.
My nephew will be 26 months at my wedding andI feel the same, he might be too young to participate. If by next spring I feel differenly, I will definitly ask for him to be part of it.

maybe the kids other grandparent could bring them and take them home...

I think its nice if all the kids are involved...but I understand your dilemma