Hello Kiramere
and welcome back to Rescreatu
There's no way to deactivate an account but you should send in a support ticket explaining how your sister messed with your own account/ why you have two, staff may be able to delete the old one :)
If you scroll to the bottom of your page you will find a link
"> Support tickets"
simply click that and explain your situation in the "Open support ticket" text box and send.
:)
( don't worry about the "Importance" number ^^)
Just in case you haven't already please have a look through the rules, found in the link below
^^
This FAQ page is also very, very helpful
but never be afraid to ask questions, every one on Rescreatu is very friendly
:D