Wow that's so.. horrible. You need trust to have a friendship (in my opion) I'm sorry. covered.in.cake's comment took the words right ot of my mouth.^^
I'm going to assume since you were lectured on this 'secret' that it wasn't something like a new crush or something. If I'm wrong, I'm sorry.
But I'd ask your friend why she felt the need to tell your brother, who then told your mom. Maybe your friend didn't think your brother would confront you- or your mom-about it. And if you told her something that was life-or-death or could potentially hurt someone, she might have thought it best to tell someone else in a way to get 'help'.
If you want to talk, PM me. And i'm not just saying that to be nice, I actually mean it.
i really appreciate it !