Plain old

"Hey, Lexi. I have a little surprise for you in the basement" You say having no better idea you decided to lock your step sister together with your cousin.

"I don't have time for that." She answers. "Can you go away? Don't ruin my evening."

"OK, I'll go, but first, a surprise." You answer, grinning "Believe me you're gonna remember this surprise for a very long time."

"This better be something interesting." She says as she goes to the basement just to get rid of you faster, you follow her preparing to jump to action. As Lexi opens the door she sees naked and bound Sara and only manages to say "What the fuck?!

"A very good fuck." You bark and roughly push Lexi against the basement wall, "And now I'll have another." You add as you grope your struggling sister's ass.

"Are you insane?! Get your hands of me!" Lexi isn't a weak girl, neither is she submissive, but you are stronger and manage to keep the struggling girl against the wall with one arm, while you use another to quickly strip your target from the waist down." I'll kill you for this pervert!" She screams once again with another attempt to get free from your hold just as you are distracted by removing your pants.