Why does my crush's dad seem to hate me?

So I'm 17, and I have a crush on this girl who is 18, and I'm pretty sure she likes me back. I've talked to her only a few times, but I always catch her looking at me. I'm also friends with her little brother, who is 14. But it wasn't until a few months ago that her dad started being extremely unfriendly and cold towards me whenever I'm around.

I'm still friends with her brother, and his dad seems to be fine with me hanging around him to play video games and stuff. But whenever I happen to talk to his sister, her dad gets this disgusted expression on his face and grunts very abruptly when I try to ask him a question. I haven't shown any obvious outward signs that I have a crush on her, so why is her dad acting like this when I'm around her, and not her brother? Her mom seems to really like me, so what's the deal with her dad?
Why does my crush's dad seem to hate me?
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