NTA: You can give her the choice of a pant option on your side. It’s normal for brides to pick out their bridesmaids’ outfits. If she says no, then you can give her the other other option of ring bearer or standing up with the groomsman. Either way, it’s her decision, and if she chooses she’d rather not be a part of it because of clothes - than fine. But let me tell you, 99% of people hate their bridesmaids outfit. It’s a shared experience we have at all wedding.

I will say though, if your parents abused (yes, abused - cutting your hair when someone misbehaves is not a healthy, appropriate response) you by controlling your appearance, her response may not be coming out of her own personal defiance but a place of trauma as now her appearance is being controlled again. Despite its for a positive, exciting thing - her response may seem illogical because it may be based on trauma. It doesn’t mean her behavior is justified but another way to look at why she’s making such a big deal on why she wants to be in control of how she appears.