The Estermonts are one of the great families of the Stormlands, and are particularly prominent because Lord Estermont is maternal grandfather to the King Robert and lords Stannis and Renly. Lord Estermont is very old, and has many offspring, with at least two sons (Ser Eldon and Ser Lomas) and several grandsons and even a knighted great-grandson.
Ser Lomas Estermont and his son Ser Andrew are part of Stannis’s household on Dragonstone. Ser Andrew in particular had been Stannis’s squire for some years.
Information about House Estermont that reveals spoilers from the books.