@yeshisama That would just make all childhood friends to lovers incest lol.
But they both have stated multiple times, despite knowing each other since they were kids and their parents having been married, that they don't see each other as siblings nor do they treat each other as siblings either. They're more akin to roommates whose parents happened to get together at this point honestly.
Heck even in this chapter Ran thinks "We're not really siblings..." when attempting to describe her relationship with him too. Also they weren't even raised together either. Yustav got sent off to boarding school (seemingly shortly after the parents married but just before their younger half-brother would be born) for several years of their childhood and it wasn't until the parents died in an accident that she even saw him again at the beginning of the series. And its implied they didn't speak during that time as well.