How do i disown my step sister?
You don't "own" her! So, unless you are her "legal Guardian", just ignore her!
just stay away fronn her as nnuch as possible and nnove out if youre over 18
Marie And Alan
You don't. Just get on with your life.
Sell her to the local crime boss.
Disowning is simply the refusal to acknowledge or maintain a connection with someone. So... do that.