He was once married to the Duchess of Elath, with whom he had two daughters. However, twenty years before the start of the story, his wife and eldest daughter died in strange circumstances, leaving his younger daughter Cayleigh the Duchess of Elath. According to Lucille, he pressured Cayleigh to marry young, to Earl Fabian, in order to ensure Elath's succession.
According to Fabian, he and Erwin forever argued on who was at fault for young Cayleigh's death in childbirth.
Since his first wife's death, Erwin has remarried a noblewoman much younger than him, and had a son named Connor, aged fifteen. Connor may inherit his father's titles in-game depending on Elodie's choices.
In Week 22, Elodie can select Erwin as Adair's guardian, which may not be good for the boy's wellbeing. If Elodie is not already engaged, he will suggest that she marry his grandson.
He also gets a small mention during Week 17 if Elodie chooses to dance with "Someone already married".