Please help me! Guys and Girls (serious answers only please)?

ive been best friends with her for 9 years nearly! i love her to bits we have been through absolutely everything together tell each other everything! i would never let anything come between us! But something has! her brother! she knows ive liked her brother for years and he told me recently that he liked me too! on friday i stayed the night with him and she told me she was fine with it! i had a drink with her and we were fine we had a good time! i stayed over on the saturday but she didnt know, and i stayed again last night with him she didnt know until this morning that i was even in their house until my mam came to her house for me! i mean we have been best friends sinse we were eleven and shes never had a problem with me liking her brother and even said she was fine with it on friday when she knew we were involved ually. But today when she knocked on his bedroom door to tell me my mam was there she was fuming! she told me she never wants to talk to me again and im absolutely gutted! ive tried texting her and apologized about a million times! ive even told her i will stop seein her brother because our friendship is more important! but i really like her brother, i have done for ages and we get on great! hes not the type of guy to want to have a girlfriend, hes a bit of a player and has had with loads of girls! But this doesnt bother me and he wants to sit in with me all the time! i really think we have got something! but i dont know what to do! my friends ignoring me and im literally heartbroken we have never had an argument before in the whole 9 years of knowing her! we were so close! i dont understand why she is so angry though as she knew i had stayed with him on friday and said she isnt bothered! Am i in the rong. do i stop seeing him for the sake of our friendship? why is she so angry with me i really cant help my feelings. ive told her all these things and she wont answer! i just want to sort it out so that i can be happy with the both of them! she knows she is more important to me than any guy! ive thought about if things go wrong with her brother and maybe she is worried i wont go see her anymore if it does but i would, i couldnt erase her from my life because of a bad relationship! what do you think she is angry for ? please help x