Imagine: Isaac thinks you’re hot but you’re Stiles’ sister, so Stiles get overprotective over you and annoyed at Isaac.
“I’ve told you thousands of times-” Stiles started with his eyes burning with anger, he didn’t like the way Isaac talked about you or looked at you. It made him sick that Isaac was trying to get together with you- his (Stiles) only sister.
“I’m pretty sure you haven’t.” Isaac interrupted with a smug smile spreading over his lips.
“Will you just shut up and stay away from my sister?” The dark haired boy asked with his hands running through his hair and eyes glaring at Isaac. “Or I swear, there will be consequences.”