[For a second Mabel's just stunned. It's always a little difficult for her to see siblings who aren't as close as her and Dipper, but she's gotten used to it. Sometimes brothers and sisters just don't get along as well as they do. But there's a huge difference between a brother who calls his brother names and one who's actively trying to ruin his whole life!

The shock doesn't last for more than a second before she gets angry. Mabel jabs her spoon in the air, as if Killua's brother is right there and silverware is any kind of effective weapon at all.]

He what!?! What the heck, your brother is the biggest jerk! [Now she's waving the spoon around for emphasis, her voice just a little big shoutier than strictly necessary.] What a huge liar! You'd only hurt your friends if you did what he wanted, and like he'd know about friendship and stuff if all he does is hang around with creepy people and try to make your life suck!