For a 5yo party I would either invite all the girls in the class, or the whole class, or just a small group (eg. 5 closest friends) rather than inviting 15, which I imagine is either 2/3 or 3/4 of the class. That means that only 5 - 8 kids in the whole class don't get to go. I would guess that at 5 most children might have a handful of really close friends and the rest of the children in the class are probably in the same level of friendship (except for maybe one or two children which might not be much liked by the other children perhaps).
So, in the case of only 5 or so children in the whole class not getting to go to a party then I would definitely invite the twin brother of the girl you are inviting. Or I'd cut down the numbers drastically.