Your name on your father's bank account gives you authority to access the funds in the account only for his benefit. Use of his funds for your benefit or for the benefit of someone other than him would be improper. If you intend to access funds for purposes other than for his benefit, then the Court could require you to prove how much of your own money that you contributed to the bank account. If by "legal custody" you mean that your sister has become your father's legal guardian, then she has responsibility and authority to assure that he receives needed food, clothing, shelter and care. If she has become his conservator, then she has legal authority over his finances.
