bm shoes debacle

Need some help/opinions! My baby brother is getting married in April, his Fiancé and I get along really well and I’m her maid of honor. Her little sister(17) is a bridesmaid and is being kind of mean about the whole wedding. Didn’t want to wear the dress bride chose, didn’t want to have her nails painted, didn’t want to wear makeup. Now, earlier in planning the bride said all the bridesmaids could wear either cowboy boots or nude color heels. She changed her mind today because 7/8 of us are wearing heels and told her little sister she would like her to just go ahead and wear nude heels. And her little sister is flat out refusing. She said she is wearing boots and doesn’t care that it is supposed to be the brides day, she said heels make her knees lock and she will be uncomfortable. Bride said she could wear flat nude sandals instead if her knees are the concern but not boots as she thinks it will make the photos look off. I told the bride it was absolutely up to her what we all wore and her sister should be understanding. She even told her to feel free to change into her boots after the ceremony; for the reception. I don’t feel like the bride is being “bridezilla” or overly picky at all. What do y'all think?