Posted:
@Krishna,
Yes, you're right that Dashrath's miseries were the cause of his polygamy. Shri Ram also wanted to teach humanity that one wife/one husband is the symbol of true love in a relationship. It's hard to love your spouse when they're unfaithful to you.
About Shanta I am not sure. You're right that neither Valmiki nor Tulsidas have her in their epics, but Ramayan has been written in 300 different sources so it's possible that Shanta was Dashrath's daughter. That would make Sage Rishyashringa Dashrath's son-in-law, and I think one of the major sources has referred to Rishi Shringa as 'a relative' of Dashrath. We need to read the epic carefully for such tiny references, because they can teach us a lot of new things we never knew before. I doubt that the saints who wrote Ramayana throughout the years would make a mistake in referencing Shanta as Shri Ram's sister. It's also written in the Bal Kand that Dashrath specifically wanted sons, not children. He expressed discontentment to Vasishta on the fact that after him the Raghukul would end. Back then women did not rule kingdoms so only men could succeed the throne. So Dashrath wanted a son to become King after him. He happened to get four sons because the queens divided the payas instead of one consuming all of it.
Even I love Maharaj Janak's character a lot. He also practiced monogamy because despite being a King, he was a saint in mind and had no worldly desires. Whether he or Kushadwaj had sons or not is unclear, but if they did not I don't think they had any desire for one.
I don't know about Sita or Urmila ruling Mithila after Janak. Remember, Sita was exiled soon after the Rajyabhishek of Shri Ram and afterwards she returned to Earth. She was hardly in Ayodhya let alone in Mithila. As for Urmila, I don't know whether she ruled Mithila but it's possible that one of her sons, Angad or Chandraketu, ruled Mithila. Angad and Chandraketu were given kingdoms to rule by Shri Ram, but I don't know whether those kingdoms included Mithila. Even if neither ruled Mithila, it's possible that it went to another king after Janak. After all, that happened to kingdoms which had no King.
Sita Swayamvar.
Kyunki Saas Bhi Kabhi Bahu Thi 2
Mannat Har Khushi Paane Ki
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