Yes, you can transfer the ownership of your account to your sister's name. Both of you should fill in and sign the Change of Ownership form at https://www.tpg.com.au/forms/Change_of_Ownership_Form.pdf?ver=072716 and email it back to us at firstname.lastname@example.org.
Any existing contract on the account will be continued after the change of ownership. There will be no extra cost for the change of ownership.
If you need further details about this, don't hesitate to let us know.