The blaze started early Sunday morning in Vallejo and was immediately considered suspicious.
Police say physical and electronic evidence as well as witness and victim statements led them to their conclusion.
Mau Dao's death is being classified by police as a suicide that led to two homicides.
"This was a tragic event for all involved. Incidents of this nature are exceedingly difficult for the family, but also for the community and its first responders," Vallejo Police wrote on Thursday.
Dao's other 11-year-old daughter was able to escape the blaze. She was hurt but is expected to make a full recovery. Police say she is now staying with her father.
"It is horrible how they died. This is not what they deserved," said Angelo Galvez, a friend of 14-year-old twins Trinh and Tram Tran.
A GoFundMe has been set up for the family.