My step-sister, who I'm not particularly close with, lives out of state and is planning on coming into town for the wedding with her two children. Her husband won't be joining her, for no reason in particular other than that he will avoid being around her family at all costs.
We in no way anticipated that he'd be coming to the wedding, but we obviously included him on the invitation. My dad has since called and informed me that my step-sister was wondering if she could bring a cousin of hers in her husband's place. I know this cousin and like him a lot (he's my step-cousin, if that's a real thing, and with my big family, is a bit too far removed to be invited). But the idea of giving her a "replacement" date is rubbing me the wrong way. Her older sister is single and will not be bringing anyone. Only our bridal party and guests that are married and/or in serious relationships have been given a plus-one. We have a handful of guests who can attend but, for one reason or another, whose spouses or SO's cannot -- and of course they're not expecting to bring someone in their place.
While initially telling my dad that it probably wouldn't be a problem but I'd have to check with my fiance first, (I'm a softy and always cave on the spot), after I thought about it for a few minutes I called him back and told him that I'd really rather wait until the rest of the RSVP's rolled in before committing either way. Our responses are due in 2 weeks and with about 75% of them already in, we've had an overwhelming percentage of yes responses. We planned ahead and made our budget flexible to accommodate a head count higher than what we anticipated, but we're probably going to have at least 20 more people attending than we expected, and obviously don't want that number to continue to grow. So it seems a little unfair to me to let my step-sister bring someone that wasn't invited, especially when we're certainly not in the position to lend that luxury to anyone else.
I'm just wondering what other people's thoughts are on this. Is it fair of me to want to wait and see? Or should I just let it slide?