...your shapeshifting girlfriend just turned herself into your own younger sister in a shining cute blue prom dress.
"So, like, what do you think about this dress, big bro?" your shapeshifting girlfriend/sister asks you in a high pitched, bitchy voice while moving her new dainty hands, tipped with fitting blue nail polish across her new curves. "Because I think this dress fits your sis I mean..., like, me super well. Teehee..."
Your jaw drops as you look at her. Not only does she look absolutely identical to your own sister, head of her high-school's cheerleading, she looks drop dead gorgeous in that blue prom dress... You have never thought of your sister this way before, but here you are now. Noticing your shock your shapeshifting girlfriend/sister takes advantage of the situation, and starts walking towards you slowly, swaying her hips and leaning towards you until you meet her light blue eyes, lacking of betraying any sign of intelligence. Could she have copied her eyes so perfectly, or did she actually change mentally into a bimbo like your sister?
She closes you mouth gently with a dainty finger and kisses you, and then asks "Bro? Like, are you okay? Did I make you shock?"
"N...no, not at all, sis. I mean... babe. It's just that I saw you look kinda cute on this dress." you awkwardly answer her that your shapeshifting girlfriend is now your bratty, but cute sister.
"Teehee, thank you, bro. And y'know, it's like, fine to call me sis. I just wanna, like, try something a bit kinky that's all." your shapeshifting girlfriend/sister smiles as she gives you another passionately kiss. "Anyway, bro. Like, what should we do next?"