yeah thats fair, I forgot the canon sister part. I have zero interest in incest so in my game in my head they're just childhood friends that did stuff like went camping together (in S1).
Let's be real looking at this game narrative without the family aspect is detrimental to understanding the real narrative of the story, so yea understandably without it you'd be missing some critical connections that make the story more sensible.
I get where you come from, but yea its obvious that Ocean is writing this story based on the family narrative so understanding the dynamics between characters pretty much depend on it, sure its even under that premise pretty weird, you know with all the incestuous stuff related to it.
For example why would the relationship between the MC and Leia be a problem if they are merely childhood friends? No problem there logically, the fact that it was an issue can only be understood under the premise they are twins and thus is taboo for them to be involved together.
Similarly why would he'd been forced to be in a close relationship with Katie if she isn't his sister, so the family dynamics with Helen favoring him and his relationship with Leia and Katie being basically jealous and trying to push in only really make sense under a familial setting. Because you can chose your friends, but you can't choose your family, so the only reason that him and Leia would have to deal with Katie is if their is a familial context to the story.