I said 'other' as well. It's mostly a NO but you could have her sit in on a couple shots if you want too and if it's not going to cost anything extra... and if it's viable, let them have one taken together too.... but don't assume that cost if it's extra, let them buy those themselves as extras.
I also agree with whoever said you can always choose the main family one out of the shots without her in it... I know it sounds a little cold, but I lived with someone before we got married, (though we've since divorced) and I wouldn't have dreamed of getting into a formal family photo at that time. Even AFTER marriage, it's reasonable to have just the mom, dad and actual kids, no spouses let alone girl or boy friends.
Of course, my brother and his SO aren't married, but they've been together so long she'd definitely be included now. (their reasons for not formalizing things are their own!) This isn't a simple thing, but it's your decision not his. Your cash, your choice, but if getting him to the studio means having her in some of the shots, have her come along. ...otherwise this could turn into a power struggle with him being absent.
Oh, and warn the photographer ahead of time!!!!! Just so he/she understands and doesn't try to sweep her into every pic. Trust me, photographers are used to dealing with photo-day crazyness.
