They are accused of the murders of 18-year-old Yanelly Solorio-Rivera and her 3-week-old daughter, Celine Solorio-Rivera.
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The two victims were killed on Sept. 24 in their southwest Fresno home.
Yarelly is Yanelly's older sister, and Arroyo-Morales is Yarelly's boyfriend. The two suspects were arrested on Nov. 9.
Prosecutors revealed Monday they believe Yarelly is the one who fired the gun that killed her sister and infant niece.
Yarelly and Arroyo-Morales were each charged with two counts of murder and a special circumstance of multiple murders.
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Yarelly is facing two enhancements of "personal and intentional discharge of a firearm causing death."
If convicted of all charges, both Yarelly Solorio-Rivera and Martin Arroyo-Morales face life without the possibility of parole or the death penalty.
The District Attorney's Office is deciding whether to seek the death penalty and will make an announcement on that decision later.
Solorio-Rivera and Arroyo-Morales are scheduled to be arraigned on Nov. 15.