And nobody has consulted an attorney. Sigh.

Your father is a creditor of the business. Whether or not he can sue anybody successfully depends on the terms and conditions of any contract that your father entered into with the business and/or the two principles.

Your husband can, obviously, sue his brother.

You have no standing to sue anybody unless you are a principle of the business or a creditor of the business. Apparently, you are neither.

You or your father, or both, may report the embezzlement to the authorities and see how that goes.

Something else that your family needs to consider. You wrote that the brother left the business and/or was fired by the accountant. That, unfortunately, doesn't eliminate the brother's ownership of a share of the business unless he signed something that relinquished his ownership.

You're right. It's a mess. And, frankly, it's your husband's deal with his brother. If your husband doesn't want to sue his brother and your father doesn't want to sue for breach of a loan contract (if there even is one) then they both kiss their losses good bye, move on, and try to recover as best as they can.