Well Brian really wasn't running his construction business day to day while he was constructing the lake house, was he? True he did leave for a couple of days a couple of episodes ago but it appears for the most part he has been at the build site. I think this build was what got Sarah so overly involved in the business...I didn't know she had run a ballet studio until I watched this show.
It seems to me that he has a great team who does most of the day in and day out running of his business. After all, he is on an island thousands of miles away while he is building the resort.