Dude, let's make it clear, the cash deposit by you in your account in USA which you'll show as POF doesn't require any letter from your father. Now the important part, the funds from your sister's account to your account was a bank transfer? If it is, refund that money to your sister right away. Then ask your father to make a gift deed and send that money into your account directly from his account.Its in USA. I deposited cash in USA and another sum was sent from my sisters account in usa.
Please do not complicate your POF as you've already done it. Keep it simple.