Deleting an account from single user mode

okay... I have a bit of a problem.

I made the mistake of creating an extra admin account on my sister's computer using single user mode and now I can't get rid of it. I need to either delete the account from within the account, or delete the account from single user mode.

The system has two accounts at the moment. One is my sister's account and the other is the one I just made.

I also cannot reset the password on my sister's account I want to avoid alerting her to what I've done.