Boyfriend keeps flirting with my sister but says he doesn’t like her. What should I do?

My boyfriend is constantly flirting with my sister on and off. He talks about her to my face about her being prettier and her having her life together more than me and when I react to it he says I’m jealous of her. I’ve lost so many guys to my sister and feel like I’m going to lose another one.

Boyfriend keeps flirting with my sister but says he doesn’t like her. What should I do?
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