My sister would like to join Subeta, but I know that it's not impossible for people using the same IP address to be mistaken for the same person. If my sister makes an account, is it likely that this will happen? What can I do to prevent our accounts from being banned?

You can make a ticket listing both of your usernames and they can make a note on both of your accounts saying that you are two different people.

Me and 2 of my children play, and I sent in tickets when they joined. We have had no problems at all :)

Oh! Thank you! How do I send in a ticket?

Via the Help link in the left side bar.

Thank you. (: