NTA if you only ask once and make it clear to her that her answer does not impact her position in the wedding party, and prepare for a negative response as she might not like you asking. Also only tell her it won’t impact her position if you’re 100% committed to honouring that

I do however think you need to look at your own discomfort here, as well as the importance your family seems to place on attractiveness, and remember that at the end of the day that her bodily autonomy and beliefs are more important than the aesthetic you want for your wedding. While it is obviously an incredibly important event, it is also only one day while your relationship with your sister is ideally forever. I think you can already see that based on your post but just an important thing to keep in mind