The blaze started early Sunday morning on Georgia Street in Vallejo and was immediately considered suspicious.
Police say physical & electronic evidence as well as witness & victim statements led then 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, " wrote Vallejo Police 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.