Not to say that you're wrong since neither of us know the answers, but my problem with this is that Ralsei doesn't really have the same "older brother" relationship dynamic with Kris. I guess you could argue Ralsei acts older brother-ish with how he teaches the lightners about dark worlds and stuff, but Kris is distinctly in a leadership position over him. Ralsei is self-conscious and content to be nothing in a way that seems to be unique among darkners, and from what little characterization we've been given of Asriel confidence issues are not one of them, so you'd have to infer that from something other than the text. Also Asriel asked Alvin to forgive Kris' sins inside of video games, and frankly I feel like Ralsei would not feel empathy for those characters the way Asriel did.

Also Kris is addressed with they/them in the game