Answer:
The sanctuary of the ancestors of Casa Li refuses to light up on Lingsi's sixth birthday because Mrs. Li did not fulfill the promise made by Lingsi's mother.
Step-by-step explanation:
When Lingsi was sold as a servant to Mrs. Li, when she was still a baby, her mother made Mrs. Li promise, in the name of the honor of her ancestors, that when Lingsi turned six years old, Li would allow her to learn to read. Mrs. Li had no intention of allowing Lingsi to have access to education, but to be able to buy the girl she promised, lip service, since she did not intend to keep the promise.
However, on the 6th birthday of Lingsi, a new tutor was hired to teach the son of Mrs. Lí. At that moment, Mrs. Li went to light an incense for the ancestors, but the incense refused to burn, no matter how much she tried to light it. That was when the tutor said that this was happening because someone in that house disrespected their ancestors with an unfulfilled promise.