Eh, people get weird about weddings... especially where kids are concerned. ESPECIALLY if you and fiance happen to be CFBC. I feel you, because my partner and I have also decided we do not want kids. This, for some reason, drives our relatives with young kids crazy. I think a little of it stems from jealousy. That's not to say that they don't adore being parents and having children - but let's face it, having kids prevents you from doing a lot of the things you used to do before kids were in the picture. Your future SIL is showing you her own jealous feelings by trying to insinuate her children into your wedding AND by also going on and on about the honeymoon - she is positively green with envy, I think!
Take a deep breath, but be firm. If you don't want her kids in your wedding, you are under no obligation to. Maybe ask SIL exactly why she feels so strongly about it?
Good luck to you!