He didn't need any preconceived ideas about his little brother or sister.
Prenuptial agreements are created for people who approach a marriage with the preconceived idea that it may not succeed.
The only problem is that it all doesn't fit into your preconceived plans.
The whole plan was based upon defective information and preconceived ideas; it has gone down to history as a classical example of bad generalship, and its author Weyrother, who was perhaps nothing worse than a pedant, as a charlatan.
Moreover, Froude's characteristic desire for picturesque effect, unchecked by any painstaking accuracy, led to his reading preconceived impressions into his documents.