Derek couldn't stop looking at you while Stiles was trying to lecture him about how he should stay away from you. You're his sister after all. But you knew that whatever Stiles said wouldn't stop Derek from doing what he wanted to. You laughed from across the room, laughing at how Derek wasn't interested in what Stiles had to say, not even in the slightest. Before long you couldn't control your laughter and Derek lifted his forearm, showing you his fist, an unamused look on his face.
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You laughed even harder at his subtle way of saying 'fuck you'. Stiles turned around to face you, his mouth wide open in annoyance.
"Y/n, will you shut up? We're trying to have an adult conversation here." He snapped, making you stop laughing. Your mouth formed a thin line, your eyebrows furrowing and your eyes narrowing as you crossed your arms over your chest.
"Adult?" You asked, trying not to laugh again.
"Yes, you don't count as one, considering you've been fooling around with this-" Stiles began but cut himself off as he turned around to look back at Derek slowly, realising he had almost insulted him.
"What am I Stiles?" Derek asked.
"I-I- nothing," he answered quickly.
"You're mine!" You piped up, making both of them turn to look at you.
"What?" Stiles asked, shock evident on his face as you walked up to Derek, not breaking eye contact.
"He's mine, Stiles. And nothing you say will deter him from being that." You said more quietly this time, your voice becoming a little deeper as you reached Derek's body. You wrapped your hand around his forearm and turned to Stiles. "You should leave, brother."
"Oh my god." Stiles breathed as he shook his head. "You're unbelievable."
Stiles left the loft as quickly as he could. You shared an amused glance with Derek before you kissed him.
