Make her an Alpha

As good as it felt, there was no way you were damning your own twin sister to life as a Gamma just so you could cum in her mouth.

You grabbed her hand and gently pulled her to her feet. Without even thinking, you threw your arms around her, pulled her body against you and started kissing her. To your surprise she kissed you back and her mouth seemed to be just as good at that as it was sucking cock.

In the corner of the room, Dr. Grant has undone his khakis and is now stroking a massive white cock. He is even larger than you.

"Ready to be an Alpha?" you ask.

"Uh-huh," Marcie says, sounding out of breath.

She turns to the exam table and bends over it with her big ass pointed at you. She looks back over her should at you and smiles, and you realize that sister or not, in that moment she may be the sexiest girl you've ever seen.