Is an email agreement binding when neither of us fully understood the situation?

If she was your ex-stepmother before your father died (divorced), then I'd say that there's no contract here, and it isn't even a question of mutual mistake of law or fact:  there's no consideration for the agreement.

"Consideration," in this sense, is a lawyer's term of art.  The law requires consideration on both sides of an agreement, to be binding.  Consideration can take many forms, other than something of value changing hands;  it can include agreeing not to sue someone, or settling a dispute.

In your case, though, the agreement gave you absolutely nothing that you didn't have, if your father was divorced from her, because the law deleted her name from the IRA when the divorce was final (which is why the bank decided this as it did).  Email and telephone conversations can indeed form a contract, but that's not the critical issue here.