Crystane Corrino and Tara Corrino were the first legitimate female births to Elrood Corrino IX. This made them royal princesses and potential heirs. They are notable as the elder sisters of Edwina Corrino, who married into House Richese via Ilban, and having a daughter Helena Richese who married Paulus Atreides giving birth to Leto Atreides, who would in turn give birth to Paul, the central character of the original Dune.
Crystane, Tara and Edwina were the daughters of Yvette Corrino (maiden name Yvette Hagal). Crystane and Tara were born at some date previous to 10070 AG when their little sister was born, though the specific date for them is unknown. It is unknown if either Crystane or Tara married or if they had any children, or when they died.
Crystane and Tara were not the first daughters born to Elrood. Their elder half-sister Wensicia Mutelli was born to his older first wife Barbara, but Wensicia was born a year prior to Roody and Barb wedding, so she is a bastard and not an official princess.
Crystane is presumably the elder sister since she is mentioned leftmost on the Corrino birth chart.
They would have been born at some point after 10036 AG, which is when Yvette married Elrood. It is likely much later, as Yvette was only 12 when she married him and they may have waited before consummating it or carrying any pregnancies to term.