I agree 100 %... Edward was the oldest of 2nd wife Jemima... he was the oldest child of last wife, not first wife. If he were the oldest of all 12 children, then he could have just said oldest, but he did say:
"Eightly: I give and bequeath unto my oldest son (by my last wife) Edward, one horse creature to be delivered by my executor after my decease." and
"Ninthly: I will that when my youngest child comes of age that my estate both real and personal (except what has been and is hereafter otherwise willed) be equally divided among James, Thomas, William, Abraham, Nancy, Susannah, Clayton, and Rachel, my children by my last wife. (N.B.) Be it remembered that prior to any of the above and forementioned dividends all my just debts and funeral charges be paid.