As siblings Sophia and Edward have the same rights to a visit from Frederick. I can see the logic of attaching greater urgency, more importance to visiting Edward because of his new bride however there are significant differences in situation which gentles our captain’s choices. Frederick has never met his new sister. It seems he was at sea when the marriage took place and I imagine he wrote a very fine, heart-felt letter to her and Edward when he learned of the event. (:D) In Emma Mr. Knightley excoriates Frank Churchill for his delays in visiting his father and new bride—the estimable Poor Miss Taylor:

"There is one thing, Emma, which a man can always do, if he chuses, and that is, his duty; not by manoeuvring and finessing, but by vigour and resolution. It is Frank Churchill's duty to pay this attention to his father. He knows it to be so, by his promises and messages; but if he wished to do it, it might be done. (E, 18)

When Emma says Frank’s excuses “seem to satisfy every body else” (E, 18) Mr. Knightley responds with:

I suspect they do not satisfy Mrs. Weston. They hardly can satisfy a woman of her good sense and quick feelings: standing in a mother's place, but without a mother's affection to blind her. It is on her account that attention to Randalls is doubly due, and she must doubly feel the omission. (E, 18)

Mr. Weston is Frank’s father so his duty to him is generally weightier than the duty owed between brothers. Frank leads a life of “mere idle pleasure” (E, 18) and does not want for money or leisure: “We hear of him for ever at some watering-place or other” (E, 18). Frank could easily have made the trip to Surrey instead of a pleasure trip at almost any time had he really wanted to do it. He has no viable excuse for the months of delay. I do not feel Frederick so recently at sea and unable to perform any duty on land would be held to the same time constraints on this particular point of etiquette so that visiting Edward first is the only right course.

When the Crofts called this morning, (they called here afterwards, did not they?) they happened to say, that her brother, Captain Wentworth, is just returned to England, or paid off, or something, and is coming to see them almost directly… (6)

Ashore, Frederick with “true naval alertness” (6) mapped out a direct route to visit Sophy in Somersetshire then Edward in Shropshire. Cheryl makes a good point—it would be a waste of time and money to travel through Somersetshire to Shropshire just to backtrack a short time later. Frederick’s idling at Kellynch “a little longer” (8) than initially planned is not unreasonable or the purposeful avoidance of duty that is Frank’s choice to travel about the country to the “idlest haunts in the kingdom” (E, 18) instead of visiting his father. I think so long as Frederick checked out the “charms and perfections of Edward's wife” (8) in a reasonable amount of time any duty he owes them would be satisfied full measure. Of course our reading this week (14) reveals Frederick has gone into Shropshire instead of remaining at Lyme with Louisa—how interesting.