My girlfriend's brother is being an jerk, how should I deal with him?

I've been spending a lot of time with my girlfriend recently, she's asked me to teach her how to tie knots since she wasn't allowed in Scouts. Her older brother is such an jerk, and keeps bothering us and saying stuff about girls being useless. Any suggestions on how to keep him away?

My girlfriend's brother is being an jerk, how should I deal with him?
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