Can't stop having fantasies about my fiancee's sister

I'm a 32-year-old man and I'm supposed to be getting married in May. My fiancee and I have been together three years and I love her, but I've been having these thoughts and fantasies about her sister for a while.

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Now all I do is worry whether I should even be getting married and it's causing me a lot of stress and sleepless nights. I only met my fiancee's sister a year ago, as she'd been living abroad.

She's now back home permanently and living with her parents, so I see a lot of her. We instantly hit it off when we met and she's a lot of fun. She makes me laugh like no one else and I look forward to seeing her more than I should. I find her so attractive and it’s killing me.