Sidra was previously engaged to Waris Ali, but when he left for Saudi Arabia last year for work, her family married her off to his younger brother Waqas, police said.
"Waris worked in Saudi Arabia and learnt about the marriage only when he returned. The suspect had developed a grudge against his former fiancee for marrying his younger brother," senior police officer Muhammad Naveed said.
He said that the accused had also business dispute with his younger brother. On Saturday Sidra was sleeping in her room while the other family members were away. Ali came there with a container of petrol and set her on fire.
He locked the room and fled. Police said neighbours alerted firefighters who reached the spot and put off the fire and recovered her charred body. Sidra's family told police that she was a few months old pregnant.
A murder case has been registered against Ali and his other family members on the complaint of Sidra's father. Police are conducting raids to arrest the suspect. Some family members of Ali have also been taken into custody for interrogation.