Watch Arthdal Chronicles Episode 7 Online with Subtitle: Tagon's Ambition and Tanya's Revelation
It's sort of explained in episode 4. If I recall correctly, the subtitles for Rottib's lines in episode 1 referred to Eunseom's sibling as 큰아들 (older son). Unless it was a subtitling mistake, I'd say he's pretty much confirmed to be a boy.
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