Hi! I'm a lurker...and I need advice 
So my FI's cousin (M) is a groomsman for our wedding which is in 3 weeks. He's 6 years younger than us (18), and he recently found himself a girlfriend (I believe they became "official" after we sent out invites; however, I'm not totally sure on that). Apparently they found each other online or whatever, and this past week he traveled across the country to visit her. So now, he's...in love....with her. *side note: I kind of role my eyes to this, but I'm really trying hard not to be judgmental to his puppy love.
FI called me this morning and said he received a text from M saying that he's invited his girlfriend to Iowa around the time of the wedding and asking if she could come along. I immediately said no for obvious etiquette reasons which FI agreed to as well. Just now, I've received a message from M on facebook asking the same question.
Neither me or FI have responded. FI is wanting to talk to M's mom before he says anything to M. But the thing is, M's mom recently emailed me asking if all of our wedding party has dates because she "doesn't want her son feeling left out." ...this was another thing I did a slight eye roll to (everyone in the 6 person wedding party are very friendly people who wouldn't make anyone feel "left out"). I explained to her that no, there are 2 other people in the wedding party that don't have dates (my brother and sister). But anyways, I think she'll try to press the idea of inviting this girl so M doesn't feel "left out."
I think the sole decider for this is my mom, being that her and my dad are the main contributors, money-wise. I think she'll just say, sure, what the hell, but my gut tells me we're riding the fine line of being over-booked for our venue, and every "no rsvp" that we get, will help us with this issue.
Should I go with my gut and say no?...what if this girl still ends up flying to see M during the time of my wedding anyways (I don't think they have the actual dates set for this get-together)? I just think it gets a little complicated with air-fare involved, so part of me just wants to allow for me...despite the fact that I (and FI) find it really annoying.
Sorry this is stupid long, but the whole thought of it irritates me, and I just wanted some un-biased opinions on it. Thanks!