of course, it doesnt HAVE to be relevant, but lets face it... i dont know about you, but i cant remeber any story, novel, book, movie, series, game or whatever where its mentioned that a main protag knows about the fact that ( at least one part) of the parents are not their biological ones without being relevant later on. There is just no point in it to mention that, doesnt add anything to the character of Lena.Well, revealing that Lena is not related to her dad doesn't have to make her biological father relevant (Axel also has "piercing cold blue eyes" but he's not Seymour's son), but might hint at certain Patreon-safe opportunities later. Also, the game early on mentioned Lena's brother who so far didn't make an appearance but might play a role later into the story, and if it turns out he's only her step-brother (being the son of her not-father before he married her mother), then, well, implications are clear. It could go either way and depends on what Eva plans to do with that. But yeah, this chekhov's gun is hanging on a wall to be used later.
and yeah, as you said "this gun will be used later" iam absolutely sure about it, same goes for the troublesome brother... he will play a role too.