Future sil wedding date

Okay so, I've already discussed with my sister who is going to be my Matron of Honor about this but I'm lost at finding the right words to explain to my fiance about why I don't want our previously decided wedding date.



Here's the story....his sister is getting married this year 2021. I totally had the wrong date down in my calendar for her. So when he and I were deciding our date, I thought her wedding was a week before. Now he and I are getting married in 2022 but the date we picked is a day before their date.....so it would be the day before their 1 year anniversary.
The moment I realized this....I immediately talked to my sister because I thought that it may cause conflict (I have Autism so some times I get social things, some times not. I wasn't sure if this was one time I was or not.) My sister agreed with me that since it's their first year anniversary it may not be okay with his sister and her fiance.
I brought this up to my fiance but he thinks I'm overreacting and overthinking. I don't know what to say or do. 😭😭😭
I figured the date would be the least stressful thing to deal with. Lol.