Responses
I feel you. Japanese and Korean media tend to be ok with those sorts of emotionally incestuous relationships. The brother/sister con archetype has been around for a long while. For some reason they portray it as something cute or endearing. But as you say, it's fucking weird.
I cannot be the only one weirded out with how the majority of the FL's brothers in these stories are just straight up obsessed with them? It's all fine and cute when they're both little kids, but half the time these brothers are fully grown and their actions are hardly any different from the ML's. The only thing distinguishing them is relation and a lack of kissing. You could easily take a lot of scenes between these siblings, remove the context of them being brother and sister and they would look like potential love interests. Some of this shit be giving emotional incest vibes.
And it only gets weirder when they fight with the actual romantic ML for the FL's attention/affection- or with other brothers or their own father! This shit is weird! They don't feel like families! They feel like reverse harems where everyone is fucking related!
I'm all for in-depth, close, non-romantic dynamics, particularly amongst family (found or otherwise), but this isn't that. It's just brothers acting like they would get with their sisters if only they weren't related.