Hello jvanwinky,
Apple Support Communities is a wonderful place for help with Family Sharing and Apple ID accounts. From the description, it sounds as though you created an Apple ID using your sisters email account, and you’re looking to delete that Apple ID. If so, you can find the steps for deleting the Apple ID via the following article.
How to delete your Apple ID account
If you don’t know the Apple ID password, you would need to reset the password to regain access to the account. You can find those steps in the article below.
If you forgot your Apple ID password
After you’ve reset the password, you can follow the steps to delete the Apple ID. Another option would be to update the Apple ID username to a different email. The first article below walks through those steps, while the second article provides the steps for changing the birthday.
Change your Apple ID
Update the birth date associated with your Apple ID
Let us know how that goes!