...your little younger sister in a shining blue prom dress.
"So, like, what do you say, 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, like, super well".
You surprise as your jaw drops that you see your cheerleading Queen Bee sister never looks this hot. Not only does she look absolutely identical to your younger sister, head of her high-school's cheerleading of her group, she looks drop dead gorgeous in that blue prom dress... You have never thought of your sister this way before, but here you are!
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 adds "So, bro? Like, where now?"