Slightly frazzled, toting coffee in a takeout container, Nina Yankowitz admitted having been up until 4 a.m. — not partying but working — as she welcomed a Sunday-morning visitor to the Sag Harbor home she shares with her husband, Barry Holden. While Mr. Holden, an architect and sometime collaborator, disappeared, laptop in hand, for a conference call, Ms. Yankowitz led her guest to an upstairs living room overlooking Noyac Bay.