Dude she shared with you the 1 thing which no women will share with anyone. Just imagine the immense amount of trust and love she has for you. Secondly, you shouldn't have shared this with your brother. one day for sure he will take advantage of this.

Shes loves you and trust you. She is in a situation were she needs you the most. She is lonely and broken. stay with her. Let her see that you are there for here in her bad days. Even if you wish to break up. this is not the moment. You could talk about this to her after she gets out from her brothers sorrow.

Lastly, yes nobody ever signed up for bad things. I never signed up to get broken twice by the same girl. But look am still with her and love her the most. And my love has changed her.

Stay and if you wish to leave, then after a while.