Nobody mentions anything about the sisters, hence we don't know. Valmiki Ramayan mentions that it was prophesied that Sita would never know a widow's sorrow but nothing is mentioned about the others as far as I have read. If they say there were no widows but Rama's mothers were still alive, it doesn't make sense. Valmiki Ramayan mentions that nobody died prematurely, so aged men dying could probably be normal. Also, women could get married to much older men and if nobody died young, then with an average age limit, the younger women would still be alive after their husbands. The thing is, during Tretha Yuga (and Dwapara) it was alright to survive your husband, it wasn't blasphemous or unlucky. Those thoughts sprouted up during Kali Yuga. Those who couldn't bear the separation entered the funeral pyre or passed away soon afterwards and it was understood by the society but not enforced.
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