My 10 year olds sister friend?

Yes, that child's mother needs to know. Its a bit outrageous for some child to do that on another person's computer. And i don't know, but maybe your sister shouldn't be allowed to hang around said girl anymore. Children pick up other peoples habits and the best/worst habit to get is from a friend. Its good that your sister wasn't exposed to it. But its like maybe if she isn't allowed to be around this girl her "habits" won't reach her. I know they say it takes a village to raise a child and that ends at letting this child's mother know what happened. Things like that need to be addressed and immediately too. But take care of your own first and try to make sure that she doesn't hang around the wrong influences.