Ask your GF/Sister if she could change into somebody else

"Umm, 'sis'. If you don't mind, could you... turn into into someone else?" you ask your shapeshifting girlfriend in the form of your own sister.

"Like, okay, bro. Whatever you say, I'll like, do it for you, bro." your sister/former girlfriend answers.

"R...really, you really mean it?" you ask again.

"Of course, bro. I'm like, really mean it, dweeb. Do I have to like, answer it twice?" your shapeshifting girlfriend/sister replies as roll her eyes with her bitchy attitude like your real sister did.

"N...no, no, not at all, it's just that I have to make sure I heard it correctly." you explain.

"Teehee, you're like, such a dumb, bro." your girlfriend/sister says. "Anyway, who do want me to be next?"