Sienna is the daughter of Mira.
Mira has said Sienna is being held captive by someone in 2149. She was supposedly four years old when the Sixth Pilgrimage arrived in Terra Nova and during the events of "Now You See Me," Mira said that Sienna would be seven years old.
Sienna was born to Mira and an unknown male. She and her mother lived on the streets and her mother was an ex-con. At some point Mira was offered reward in exchange for taking down Commander Taylor and assisting Lucas in creating a portal that goes both ways. She agreed to try to take down Taylor for Sienna's benefit. It was part of the "agreement" that Sienna live with Mira's employers while Mira does her work.