Is it wrong for your cousin, who is also your best friend, to start dating and hooking up with your ex?

My cousin is hooking up and dating my ex. I just now found out. Apparently they started texting a year ago when we broke it off. He initiated it and she went for it. And they just started talking again for the past few months. She is also my best friend and didn’t think she was this kind of person. It’s not that I’m jealous or upset about it being this guy, it’s the fact that she went behind my back and hid it for a year. Think of your best friend doing this to you and hiding it from you. How would you feel?

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