Revenge.

You and Tina sat in your room, you were both now dry.

"Explain it to me again," you said,

"We're gonna turn him into a girl, basically. See how he likes being treated like he treated you!"

"Does that mean you can turn me back?"

Tina frowned, "I'm so sorry but no, contact with cum makes it permanent,"

You frowned too, you thought to how mu h you hated your step bro for locking you into this form. "Lets do it!"

"Good girl!" Tina said, kissing you, "And then after, I can show you how a girl treats another girl," she winked. And started performing the spell.