...even though she's single, invite haven't gone out yet, and we aren't giving singles a +1.
She wants to bring a random girlfriend. Both FI and I have told her that we are not extending invites to single guests (her and the best man included).
I wouldn't be surprised if she brings her anyway, at which point she will be told there is not a seat at the table for her friend.
Hurray passive aggressive family members!