I see. Thank you for the additional information, Michael.
If your father is one of the account holders and did provide consent to add your sister to the account, that would typically be enough for the bank to lawfully add your sister to the account. The bank would normally require written consent from at least one of the account holders and your sister would typically be required to sign a signature card and account agreement before she would be authorized to use the account, but the facts you have provided do not suggest anything improper was done.
If you do not wish to share the account with your father and sister, you could typically be free, as an account holder, to withdraw funds from the current account and open an individual account.
I hope this helps clarify the situation for you.
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