Should I tell her mother?

[Ashlee-Rose]

So we have these really close family friends who we've known for a long time. They have a 12 year old daughter, and her mum asked me to keep an eye out for her - I am like her 17 year old sister, in a way.

The other day she followed me on Twitter, and I went onto her page out of curiosity - I didn't know she was old enough! But when I was going through her recent tweets, I found out that she was basically cyber-bullying another girl via Twitter. (The other girl seemed like a total brat, by the way, but she lives in a different country and hadn't been bothering my 'sister' personally.) It was something about Justin Bieber, which was interesting, but the thing that got me was the cruel words and the language she was using to bully this girl. It was AWFUL - she used words I wouldn't think she would know.

I was wondering what to do about it. Do you think I should just leave it alone, and ignore it? I feel like I should tell her mum, because she could be really hurting this other girl. If I tell her mum I will lose her trust, but I really think it needs to be stopped. Any thoughts? I'm torn between being responsible and betraying the girl's trust. Help?