I say you & your brothers run a nice train on her, break your brothers heart and tell him to never do that shit again.
If he's ever unsure about another girl in the future he should let you hit it first & you will deliver your verdict after you've smoked a ciggy & showered
Seriously...
If you were my brother and let me go out bad like that I would kick your fucking ass.
Use some reverse psychology & tell him your not sure about some girl & ask him would he tell you if he knew she was a slut to see how he feel about it.