Answer:
She is sad because the narrator's return means that the moment has come when she will need to leave the house where she lived for many years and has many memories.
Step-by-step explanation:
"My Old Home," shows the moment when the narrator returns to his hometown, after 20 years, to help his family move to another home. The house where the narrator grew up is no longer of the same quality and needs to be abandoned. The narrator feels strange when reviewing the house, because he no longer has the emotional attachment to this place and does not feel that this is his house. He realizes that his mother is also a little sad, probably because she lived in this house for many years and leaving for another house, causes her to leave many precious memories behind.