So get everybody together at a restaurant and let them get to know each other. You are infantilizing your sister, by telling her how she is "supposed to" feel about someone she's never even met.

I'm not trans, but I'm gay. I have had bad experiences in the past with members of certain religions. I can't just "turn off" the part of myself that feels shame, pain, and trauma when I am around people who project a certain "energy" (being deliberately vague on the details, so I don't derail your thread).

I'm not saying you should call off the wedding, but a simple statement of support like, "I understand why this is a big deal for you. How can I help you to feel safe at my wedding?" would show you are a good ally to your sister.