OP hasn't responded to any comments (which makes me think this is fake tbh) but I honestly think y'all are both at fault.
The key components to this are -
The brother freaks you out  
Your sister feels a need to do whatever her fiance wants  
You asked her to change it back and she said no. You got the siblings involved and she said no again. You got the parents involved and she, once again, said no and got physical with you.
Am I seriously the only one who thinks you both messed up?
I don't even feel like she should be marrying her fiance if this is how desperate she is to please him. She should have just changed it back the way it was - your comfort is more important than fulfilling her BIL's little horny fantasies.
HOWEVER, once she said no the first time you should have accepted that and told her you would no longer be in the wedding.
She's right - this is her wedding. You getting the siblings and the parents involved in a bid to pressure her into doing what you want when she already said no twice is lowkey patronizing.
I can understand why you're upset but I can also see why she is as well.
You got everyone to gang up on her because she wouldn't do what you wanted...and then after three no's and after getting everyone involved you finally decide to accept the no and just come as a guest. I'd be pissed. Not enough to attack you like she did but I would be like wtf?
I also feel like you should have approached her fiance after she told you the first time that it was her fiance that wanted you to walk with his brother. His going to be your "family" so take it to the source and say straight up that you feel uncomfortable about this and would really appreciate if he could try to understand where you're coming from.
This is where the downvotes will get me but (if this is real):
My advice would be to apologize for making this into a family issue when it never should have been. However, make it clear that this is a hill you're willing to die on and you're more than willing to just be a guest or not come at all if she's not willing to stop the BIL's behavior.