Am I crazy for being mad my boyfriend because he’s too close with his sister?

Anonymous
My boyfriend and his sister have a good relationship and i dont hate her or have negative feelings towards her. But sometimes i feel like they’re way too physical with each other, like cuddling, and my boyfriend slapping her butt and stuffs. And whenever they sit together, be it a couch or a bed, they’d be together skin to skin, like my boyfriend puts his legs on her laps, cuddle or lean on each other and stuff. I’m not jealous of his sister but when i see those kinda stuff, I’m completely turned off and i feel uncomfortable talking to him. Besides, his sister doesn’t like me that much and she would often get mad at my boyfriend when he comes over, she’s kinda jealous of me when my boyfriend gives me more attention. Maybe it’s because i dont have that kind of relationship with my brothers, I don’t understand the way they are. Is my feeling relevant or am i just crazy?
Am I crazy for being mad my boyfriend because he’s too close with his sister?
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