it is not so very odd. We don't know how long their parents have been dead, but it might be that Sophia Wentworth played an important part in the life of her young(younger??) brothers. That could explain Frederick's glowing appreciation of an independent woman who's capable to hold her own.

BTW it just hit me - second read you know, and no critical editions in sight ;) - that Frederick might be extra eager to have a family and a home of his own. If he had been left hanging (around) with his clerical brother once Sopia went to sea with her sailor.
He must get points for appreciating Anne and for realizing he had lost a treasure. No wonder he was hurt and angry to realize Lady R:s opinion so easily outweighed his own.
Young men are supposed to storm and rant are they not? But he certainly ought not to have clung to that hurt pride for years...