I definitely understand why she's upset with you. You went behind her back to mess with someone in her close family. Losing your virginity is an emotional experience regardless of what people claim, and your decision to do it with her brother opened up huge possibilities for tension and drama between all three of you. Then, instead of being honest with her from the start, you proceeded to hide it and tell another one of your friends. That's just extremely immature on your part, and she has every right to be upset with you for that. I'm sure she felt completely betrayed by her "best friend" for going behind her back with her brother then again with another friend to talk about it, leaving her in the dark the whole time.