My 11-year-old sister downloads ?

Turn on the parental controls on your internet settings and she won't be able to access that material anymore. At 11, she's just really curious. You might just talk to her and calmly ask if she has any questions that maybe you could answer. She will probably be horrified that she's been found out, so if you do broach the subject be sure to be understanding. I wouldn't tell your mom unless it started to become a real problem, which it shouldn't if you turn on parental controls.