She was portrayed by Catherine Rabett in Agatha Christie's Poirot.
By the start of the story, Lydia Lee, alongside her husband and other family members, were suffered from the tyrannical, abusive and tormenting rule of Simeon Lee, who made a malicious prank near the Christmas Eve through the announcement of rigging his last will and testement.
Upon the return of whom claimed to be Pilar Estravados, Simeon Lee's long lost grand-daughter, as well as the feud cemented around the old man's precious diamond that he had claimed from his deceased working partner through his unscrupulous means, the situation cemented and the family started to hate Simeon and even, at some point, spread the hatred to each other. Lydia was one of the few, if not the only, who managed to maintain the situation and comfort her siblings-in-law.
Lydia was the first to notice that something happened to Simeon Lee when she went upstairs and found Simeon's door locked up. She then heard a loud noise like yelling inside, of what appeared to be Simeon Lee screaming. She and other family members rushed to the scene and broke through the door, only to find Lee dead in the pool.
Lydia's presence and alibi was witnessed by Pilar, who saw her approaching Simeon's bedroom. After the murder of Simeon Lee, Lydia provided testimony to Poirot and spoke out for Pilar's and others' innocence, especially when "Pilar" was exposed as an imposter and thus became the prime suspect at the time. Lydia's testimony made Poirot to deduce that Simeon was already killed before the altercation took place, because the killer faked Simeon's scream with a special balloon.
Since none of the Lee family's member had ever entered the scene, and Pilar and Lydia both provides alibi to each other, Poirot deduced that the killer is never a part of Lee's family who wanted Lee's fortune or jewelry, but an outsider desiring to seek vengeance against the depraved man.
By the end of the story, after the denoument and solotion of the case, Lydia accepted Pilar and Stephen Grant as part of her family, inviting them to a proper English Christmas Party.