I think why it works in Game of Thrones is that it’s used both sparingly and in ways that feel like an alternate-universe feudal European society might plausibl

I think why it works in Game of Thrones is that it’s used both sparingly and in ways that feel like an alternate-universe feudal European society might plausibly coin them. “Brother-by-law” and “sister-by-law” are particular examples of coinage that feel realistically archaic and foreign, yet aren’t arbitrary “smeerp” words, either.