My sister and my ex!!!!

I’m not sure why I am upset because I feel like I shouldn’t be? But at the same time I feel like my sister should know better than dating my ex! I am married now so I obviously have zero feelings for my ex but the thoughts of them together makes me sick! Would I be selfish to tell my sister how I feel about her relationship with him??

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I lost my virginity to this guy!! Dated from when I was 17 till when I was 24( a year before I met my husband)