Mandavi
Urmila
Sutkirti
These are not direct sisters of Sita
Mandavi is wife of Bharata
Urmila is wife of Laxmana
Shurtakirti is the wife of Shatrughna.
Sita is wife of Rama
Rama's father Dasharatha had three wives called Kausalya,
Sumitra and Kaikeyee.
Rama born to Kausalya
Bharata born to Kaikeyee
Laxman and Shatrughna born to Sumitra.