Edward had also invited over Roy and Riza, creating for one very big dinner party. But their dining room (transmuted to be combined in part with the living room, and furniture moved out of the way) could handle it, and a transmutation later, so could their table.
Edward takes his seat at the head of the table as his husband and the ladies serve up, while William takes his place at the other end, neatly setting up the family's power dynamics and leaving the rest of the table for others to sit where they wanted.