I think that's intentional.
They let you try her out but then she leaves your party, they offer to give her back as a freebie on your 1st store purchase.
Joke's on them though I like the younger girl better I don't need her or her sis.
Plus if you don't buy them, you can rewatch the video advertising them over and over again which is great service. Helps to explain why this is a 900MB download for such a trash game which reuses mechanics of another zombie one I played couple months ago - all in the cinematics.