Legal Question in Civil Litigation in California

Only if your mom gives you an assignment of her cause of action for conversion.

I agree with Mr. McCormick. Your mother was the victim of your brother's theft, so she is the one who has a case against him. Her claim did not automatically become yours just because you helped her out. She has a case against him if she wants to pursue it. Alternatively, she can assign her claim to you so that you can pursue it yourself. If she is unwilling to do that, then you will not be able to sue him for this debt.

Both of the above attorneys are correct in their legal analysis. Based upon my experience, your mother will not want you to sue your brother so probably will not assign the cause of action to you so that you can sue. But how did he get it out of her account? Unless he is named on the account, the bank may have been negligent in allowing the withdrawal and your mother might assign that claim to you so that you can get your money back.

Remember, no good deed goes unpunished.