Indeed, that's the one poor Sophie ended up with. Now, the reasons why FW absolutely wanted to have him as a son-in-law were three fold. On the hand, it was strategic, because the von Schwedts were actually Hohenzollerns from a younger branch of the family, which meant that if by some misfortunate Fritz and all his siblings had been struck by lightning before they could procreate, this guy would have ended up on the Prussian throne, as a prince of the blood. (Fritz has a very sarcastic comment about princes of the blood in general in his political testament.) As the von Schwedts were ambitious, giving them one of the princesses basically bound them to the royal family in FW's mind, instead of giving them reason to plot against it. Secondly, that Margrave, between loving to hunt, loving to drink beer and loving rough pranks was just FW's type of fellow. And thirdly, the very fact that Milhelmine had refused him, given how FW related to his two oldest children at that point.

He was, however, sick during the actual wedding, which meant Fritz was the one who gave Sophie away in church. And if he provided her with an authorized knight to shield her from Schwedt's abuse, I'd say Fritz must have had a pretty clear idea about what that brother-in-law was like from the beginning, too.

Like I said: among the choices she had, Wilhelmine clearly picked the right guy.