I'm sorry, but I genuinely believe there should be a "incest" tag in this context. The story begins with the main characters, FL and ML, as genuine biological twins in their past lives. Their rare blood types and the fact that FL's heart could save ML's life clearly establish their familial connection. Even then, their relationship hints at a blurred boundary.
In their next life, ML reincarnates as FL's brother, but they are no longer biologically related (this is explained through a hypnotic revelation). FL is aware that ML's soul is connected to her twin brother's, though she's unsure due to the lingering twin bond. Despite this, she treats him as if he were her twin from their first life, which is understandable given his soul's origin. However, the core of the story revolves around FL and ML's love for each other, despite being literal biological siblings in their original lives.
To me, this raises ethical concerns because their love is based on a shared soul connection from their past. Although they're not biologically related in their new life, the concept still feels uncomfortable. As someone who values souls as the essence of a person, I find this storyline involving biological twins romantically involved highly problematic.
Therefore, I feel that a warning should be present, given that the characters' relationship goes beyond the typical sibling dynamic.