Your best friend’s wedding or your sister’s wedding: Which would you choose?

Your best friend’s wedding or your sister’s wedding: Which would you choose?

"My best friend rescheduled her wedding for September and I’m one of the bridesmaids. My sister got engaged on Christmas and I just realized her wedding date is the same date as my best friend.
I don’t know what to do because I’ve already committed to being in my best friend’s wedding, but I don’t want to miss my sister’s either."