Personally, I'm not put off by the fake-incest as their relationship is just titular but, I think it's because she treats him emotionally like an older brother (even if he doesn't see her as his sister). That, and they don't really have a lot of chemistry together. (And you're right, Lucian is pretty off-putting too...)
Maybe because i've read this a long time ago but i always got the feeling that her treatment of him as her brother is just an act. I don't think she sees him anything more than a favourite character that she sneakily wants to help.
But i guess this is a matter of perspective and i totally get your take
Maybe not necessarily brother per se, but like in the non-romantic category of "relative/family". Since she's basically spending time with and interacting with his family as well, not to mention writing letters to each of them. Though I totally also get that she also is still seeing things through a 3rd person lens despite being involved. She went into adulthood (twice) not feeling much else except this for him, and I don't really have confidence that the writer can successfully make those feelings change into something else without feeling like a forced cliche. But yeah, those are just my thoughts (thanks for reading).
Tbh i don't get why ppl are so bothered by this ship. They may be "step-siblings" but in reality Fl spent like 2 weeks with the Ml lol they didn't grow up together.
But Lucian is so offputting i will probs drops it anyway