I'm with Shoo - if I understood correctly, your DF chose BW to be his "best man" and his brothers are simply attendants? I would be kinda upset if I were the brothers! Either way, I am sorry BW is causing trouble for you. Hopefully she is simply unaware of how she is acting and coming across and once she is "confronted" she will shape up!
Slightly off-topic, but maybe he didn't want to choose between his brothers? Eg make one brother the Best Man, and the other brother "just" a Groomsman. So this seemed like a fair solution?
On topic with the slightly off-topic: I've heard it suggested this way for girls. Generally, if you have one sister with whom you have a good relationship
, she should be your MOH, while friends are bridesmaids. More than one sister where there's not an obvious choice: Pick your best friend to be MOH, and your sisters are bridesmaids. That way they're on the same "level".
Not that I would necessarily do it this way, but it's something I've heard.
On topic with original: I would also ask DF to remove BW from the wedding party. She's trying to grasp at straws for a relationship
that will never be. IMO, that probably won't end well, unless a serious talking-to brings her to her senses.