You go to the bathroom

You take your decision, you can't stay there anymore or you'll lose yourself. You exit Tina's room, leaving her groans behind, and march to the bathroom. It's empty, you wash your face and casually look in the mirror. Kim looks back at you with a disbelieving expression. Somehow you almost forgot you were her; nay, it's more like you've assumed you were in her body as if it was normal.

"Oh god, this is not going to end well.".

You start thinking about the last thing you learned: shapeshifting. You guess, you feel, that in your goo form you could perfectly reproduce your sister's body; on the other hand, you can make limited changes while inside her. Actually, there's something you always wondered about: what if Kim was brunette, instead of blonde?

You close your eyes with this question in mind; when you reopen them, that's what you see:

"No way... no way... that's impossible..." You whisper, and hearing Kim's whispers arouses you even more.

"This should be illegal. I'm such, I'm such... such a bad girl.".

You quickly get rid of your panties, sit at the edge of the bathtub and start fingering your pussy.

"I'm a bad girl." You repeat, trying to keep your voice low to not wake up you parents: "I'm abusing my precious sister's body like she's a whore. I should be despised for what I'm doing. I should be... yeah... insulted... called a slut... a dirty, dirty slut, which masturbates thinking about her little brother~".

Finally, you came screaming your name. If you didn't know for sure your parents were heavy sleepers, you would totally get busted. Instead, you calmly admire your reflection in the mirror.

You're not leaving it. Ever.

"I'm sorry, sis. I'll take the lead from now on.".