In the Bhagavatam, 9th Canto, 13th Chapter, we find this verse:
SB 9.13.19 — The son of Śīradhvaja was Kuśadhvaja, and the son of Kuśadhvaja was King Dharmadhvaja, who had two sons, namely Kṛtadhvaja and Mitadhvaja.
Here Shiradhvaja is Maharaja Janaka, so called because of his emblem, the furrow, as an embodiment of his daughter being born from one. In the Ramayana, Kushadhavaja is said to be his brother. But here it is said, "Kushadhvajas Tasya Putro", meaning Janaka's son.
Even so, it would mean that either there was some mistake in copying the original sloka, which would imply that Mandavi and Shrutakeerti had a brother called Dharmadhvaja, who by extension, would be Sita's brother.
So, the question: Is he mentioned anywhere else? (Of course not in the Ramayana, but elsewhere?)