You are not in legal trouble for using the account to protect them from overdraft if you had their permission to access their account. The only people that could come after you for accessing the account are your sister and mother.
With that said, since you did access the point visa account with their permission they cannot now say that they do not owe the money because it was opened by an error. The fact is that they took steps (through you of court) to take money from that account that they knew that they would have to pay back.
This is an entirely different story if they are saying that they never gave you permission to access their account and to transfer funds from the points visa account. If that is the case then you have committed a civil violation that your sister and mother can sue you over. If you did not have permission you have also committed multiple crimes of theft.
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