My sister is flirting with my crush?

So I’ve liked this guy for 3 years and I haven’t seen him for a year cause he moved but now we go to the same college. So my friend and my sister and I hang out with him sometimes. And my sister and the guy both know I like him. My sister said she wouldn’t try to date him because she knows I like him but when I read their messages to each other it’s all flirting and sexual. She’s been doing this behind my back and she still thinks it’s a secret should I confront her or wait for her to tell me? And do I have a right to be upset?
My sister is flirting with my crush?
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