Well, not that you're asking for advice, I'll offer it anyway.
There's a few possibilities here. Either You've done something you should know about, in which case it's up to you to decide if you want to apologize or not, or you've done something you don't know about, in which case you still get to decide if it's something you want to apologize for or not, Or... it's got nothing to do with you at all, but you're safe. Meaning - she's angry at someone else, and taking it out on you, because you're her brother, and you'll love her no matter what. If that's the case, you need only love and support her, and eventually she'll work out what she needs to, and you'll be that much better off for being there.
Time, patience, and understanding might be your best friends here. And love, of course, but that goes without saying.