Why can't we be friends?

I just got off the phone with my little sister and I can tell in her voice something was wrong. She's 14 and she's just started high school at a very elite private Catholic school. Seven girls from her old middle school attend the school with her and she tells me that she's no longer friends with six out of seven. Basically, she says that the girls started talking behind her back, but act like her friend in her face until another girl from different set of friends tells her the truth. The girls who she's known since the age of six have turned on her.

My little sister is very strong-minded, so she acts as if she doesn't care and she will move on from this situation. We are very much alike where we talk big, but we are really hurting inside. Of course, I'm thinking in my head that I need to say something positive; instead, I tell her this will not be the first nor the last time girls/women will cross you. Basically, I tell her that girls are catty and this is what they do when they are jealous. I try to be honest with my little sister at all times and I just couldn't bite my tongue and try to sugar coat the meanness that girls can be to each other; especially, when it's girls who you've known for a long time. For instance, my little sister knows that I had an incident this summer with my ex college roommate and we are no longer friends due to her own vindictiveness and jealousy. I told her this wasn't first time I lost close personal friend, but its definitely will be the last. I'll admit I'm not most friendliest person in the world, but I'm genuine and I treat friendships higher than some immediate family relationships. At times, I invest too much energy in being a great friend.

I just hope that I didn't scare my little sister from new female relationships. I don't want her growing up believing all woman are shady. I want her take her time getting to know people in any kind of relationship because the best friendships are the ones that grow with time.