During the Ascent of the Lion, when Highgarden was attacked, Tristan's father died defending the city and his titles passed down to Tristan Serry.
Tristan's cousin, Elinor Serry was married off to Lord Godric Grimm. Drunk and rowdy at the marriage ceremony, Tristan made a scene insulting the lord. Godric, in return, convinced Ser Pylos Chester that Lord Tristan was responsible for the death of Lord Mathis Hewett, the father of the bastard woman that Ser Pylos loved. Without a father left to legitimize her, Ser Pylos would not longer be able to marry her, and in his rage he killed Tristan Serry in his bedchamber.
Lady Catelyn Serry, mother
Lady Margery Serry, sister
Lady Elinor Serry, cousin