Weird situation my younger sister was in. Can anyone give insight on this man’s behavior?

My sister goes to a gym. There’s a coach who clearly has some level of attraction to her based on the fact that he is always hanging around her and seems to know and be very in tuned with her gym schedule. Instead of being nice to her and making her feel included, he flirts with other girls in class to get her attention. When she doesn’t give it to him, he becomes frustrated with her but still hangs around. His siblings stare at her but won’t speak to her. He stares at her but won’t speak to her. In another class he ignores the other girls in class for her but she stops taking his classes because she thinks his behavior is inappropriate. After a while she goes back to his classes but instead of being nice to her, he makes it a point to hook up with another girl in front of her, then sits and watches her. She continues to focus on her workouts and ignore him. She’s still very professional to him, even asking for his help and rewarding him when he does his job well. He however, gives her dirty looks and doesn’t coach her. If she wants coaching in class, she has to ask him. Sometimes she’s met with sarcasm and other times hostility. His siblings still stare at her. I know my sister is shy and can be a bit of a wallflower but why bother her? It seems as if he hates her even though he’s made no attempt to actually get to know her. She’s since left this gym because of him but she feels horrible and confused and still doesn’t get why he treats her this way. Even if she’s nice to him and doesn’t ignore him, he continues to flaunt his other conquests in her face during class. At times he even makes it a point to grab another female while giving my sister a dirty look. The other girls mimic my sisters clothes and hair then follows her around which makes him seem to like them more. Why? What is this behavior? If he doesn’t like my sister, why can’t he leave her alone? If he likes her, why can’t he be nice to her? My sister is 26 and the guy in question is 29.
Weird situation my younger sister was in. Can anyone give insight on this man’s behavior?
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