I had a little cousin who screamed at me and pouted to her parents because she asked for me to give her my copy of Little Big Planet 2, and I said no.
Was some really funny stuff.
But anyways, you should give her a lecture about how the maker of the game said if you get too many bans on a blockland server, or use it twice per day or something stupid, that it will cancel your key and delete itself. If you catch her on again, when she leaves hide the BL folder and just in case, take off the key.
For minecraft, log out. When you're parents say "HENRY GET BACK ON MINECRAFT RIGHT NOW!" say "Minecraft made everybody change their passwords, and because I haven't been able to use my computer as often, I cant remember my username.
If your sister somehow remembers your username, then say "No, that's my friends account, he let me use his account for awhile because he needed me to do something. If your parents say to get back on your friends account, act like he changed his password.