As a parent I would want to know what my child is doing on the internet. You must tell your parents, clearly your sister knows her way around a computer and you and your parents need to teach what is right and what is wrong out there. Maybe what she did was just an experiment, but all the same, it is a very dangerous one.
Talk to your sister about it, tell the stories of children who have been kidnapped by sickos who looked for them through the internet. You must try your best to keep her safe.
We went through something like this with little Jess, not to this extreme, but once we sat down and talked to her about it, she understood that it was not safe.