In earlier drafts of the first film, Aunt Sarah was written as an overbearing, meddling mother-in-law, and showed antagonistic feelings towards Jim Dear. She came to the house to see her new nephew while Jim Dear and Darling were still at home, and continuously pushed for them to muzzle/kick out Lady, as she was constantly found guilty of mischief committed by Si and Am. Near the end, when Lady was found to be innocent of attacking the baby, Aunt Sarah angrily left after Jim Dear accused her cats of being cowards, and was not shown to have redemption in the end.
Having such a deep appreciation for cats, Aunt Sarah dislikes dogs a lot, and is very antagonistic towards them, being quick to blame them for any incidents that occur.
In the film, Sarah is put in charge of Lady and the newborn child while Jim Dear and Darling go on vacation. She takes an immediate dislike to Lady, and shuts her out of the nursery when she tries to see the baby. While she is upstairs, her two cats, Si and Am, cause trouble downstairs and wreck the house, claiming it as their own. Although the cats caused all the trouble, Lady gains all the blame when they pretend she attacked them, and ends up muzzled by Aunt Sarah. Luckily, Lady is able to wriggle out of Aunt Sarah's arms and run off, knowing that she will be safe.
After Lady is taken to the pound, she is picked up by Aunt Sarah, but she ties Lady to her doghouse as punishment for running away. Lady suddenly spots a rat climbing through a hole in the fence. While she barks at the rat, Aunt Sarah tells her to shush. Tramp hears Lady's barks, and she tells him that there's a rat in the house. Tramp follows the rat to the baby's room, and kills the rodent before he can harm the baby. The loud commotion alerts Aunt Sarah, who notices Lady and Tramp. Believing they want to harm the baby, she locks Lady in the basement and forces Tramp into the closet. She then calls the pound, and has Tramp taken away by the Dogcatcher.
When Jim Dear and Darling return, Aunt Sarah reveals that a stray dog, Tramp, had been in the baby's room, and that she had him taken to the pound. After hearing Lady barking, they release her from the basement, where Aunt Sarah had locked her up. Jim Dear realizes that Lady is attempting to tell them something, and so he and Darling follow Lady to the baby's room. There, Jim Dear and Darling are able to find a dead rat. They realize that Tramp had been trying to protect the baby from the rat.
Aunt Sarah is not seen again afterwards, but is mentioned in the ending as having sent Lady and Tramp some dog biscuits as an apology.
Aunt Sarah returns in the 2001 sequel. She is first seen in the opening of the film while Jim Dear and Darling are taking a stroll in town, where they meet up with Aunt Sarah. She claims she will be attending their family picnic, as Si and Am love Independence Day. She is later seen at the Independence Day picnic. Scamp disrupts the family's picnic by scaring Si and Am (who were hiding in the picnic basket, about to steal the chicken) into Aunt Sarah's arms. Scamp then steals the family's chicken, determined to prove to Buster that he is a Junkyard Dog. She then watches in horror as the other Junkyard Dogs ruin the Fourth of July picnic.