The in-laws of flight attendant Anissia Batra, who allegedly committed suicide last week, sought exemption from joining the probe on health grounds, police said.
Ms Batra (39) allegedly jumped off the terrace of her residence in south Delhi's Panchsheel Park on July 14. Her husband, Mayank Singhvi, was arrested on charges of dowry death on Monday.
Mayank's parents were called for questioning on Saturday, but they did not join the probe, the police said.
"In the case, a request letter from them was received through their counsel requesting exemption for three days on health grounds.
"A notice will be again issued to them to join investigation," Deputy Commissioner of Police (South) Romil Baaniya said.
Another officer said that they will be issued a notice to join investigation on Monday. "If they fail to turn up, we will request the court to issue non-bailable warrants against them," he said.
A family friend of the deceased demanded the arrest of Mayank's parents. Anissia Batra's parents have alleged that her husband used to physically abuse her.