In Ramayana, where it is mentioned under what circumstances did Urmila - Wife of Lakshmana die ? Because I googled and couldn't find a valid answer to it. We know that she also opposed her own husband for supporting Rama and for taking Sita to forest.
In Valmiki Ramayana, it is mentioned that no woman become widow in Rama Rajya. So I think sisters of Sita died earlier than their husbands. Please refer shloka 99
While Rama was ruling the kingdom, there were no widows to lament, nor there was no danger from wild animals, nor any fear born of diseases.
According to anand ramayan, (though it is different from valmiki ramayan) after the departure of all brothers, the sisters of sita entered fire and went to heaven (I am unable to paste the link because I have the book in pdf format but if anyone wants to read can download it - https://archive.org/details/HindiBookAnandRamayan but I pasted the pic please see