Aman Bro and everyone. In this ff I am writing everything about this topic that I found till date.
In an article I found that just like the brothers were the parts of god vishnu’s body, the sisters were also a part of goddess laxmi body.
Urmila, Mandavi, and Shrutakirti were incarnations of the wives of Adiseshan, Sudarshana, and Panchajanya, although of course those goddesses might in turn be incarnations of Lakshmi. Now I think in some Purana the wife of Vishnu’s Vyuha form Sankarshana is mentioned, so that’s who Urmila would be an incarnation of.
The wife of Sudarshana is called Kirti (fame) as told by Dushyant Sridhar in his discourse on Sudarshana Vaibhavam. So similarly Shankha will have a Shakti and these Devis would have incarnated as the daughters of Janaka and Kusadhvaja.
About their Deaths:
Uttara Kandam says that Bharata and Shatrughna’s wives also gave up their lives along with Rama and the brothers.
Adhyatma Ramayana is also very close to the above translation of Uttara Kandam and says the wives followed Bharata and Shatrughna.
About Urmila’s death I read that while going inside earth sita givve the responsiblity of her sons luv and kush to urmila. Urmila promise Sita that she will look after her sons like a mother. So urmila didn’t take jal samadhi with her sisters but stay alive to guide the throne of Ayodhya. While Ramcharitmanas is not specific about wives, it does mention that all the people who loved Rama and could not bear to live with him. So most likely Urmila would also have taken Jalasamadhi. I am extremely sorry if it confuse you but its the confusion which make these epic great and intersting.
I request all of you who are intersted in epics like mahabharat and ramayana to give some reviews so that I can research more. And please give info or ideas or any thoughts regarding this by commenting.