(CNN) Fresno police in California arrested two people this week for the September killings of 18-year-old Yanelly Solorio-Rivera and her three-week-old daughter, Celine -- and Yanelly's sister is one of the suspects, according to authorities.
The Fresno Police Department arrested 22-year-old Yarelly Solorio-Rivera and her boyfriend, 26-year-old Martin Arroyo-Morales, on Wednesday for the double murder, said FPD Chief Paco Balderrama.
On the morning of September 24, officers responded to a Fresno home "regarding a report of multiple gunshots heard within a bedroom in the house," according to a news release from police. Officers entered the bedroom where they found the mother and child "suffering from multiple gunshot wounds," said police.
The young woman was fatally shot in her own bed while holding her newborn daughter, said Balderrama. Both were pronounced dead at the scene.
"The scene was traumatic for all officers" and crime scene personnel involved, Balderrama said.
Police had suspicions about the suspects early in the investigation but detectives didn't have enough evidence to make an arrest until this week, the chief said.
Last week, police had released video from an exterior camera showing a person walking from the scene, in hopes of obtaining new information.
"Through various investigative efforts, we developed a positive identification on the individual in the video," Balderrama said.
According to police, the suspects admitted to the killings and told officers where to find the murder weapon.
"We have the murder weapon, a Smith & Wesson 9mm handgun, and we have a motive -- jealousy and sibling rivalry," said the chief.
The suspects were transported to Fresno County Jail, and both were booked for two counts of murder. It is unclear whether the suspects have legal representation.
"Although arrests have been made in this case, detectives are asking anyone with information to contact the Fresno Police Department," said police.