she was adopted when she was 10 and they lived together as siblings-ish for two years before she was left to live elsewhere for seven. but the ml says he never viewed her as a sister, always as a childhood friend with whom he lost contact until they reunited as adults. and she said that she views him more like a celebrity/bias than an actual sibling, BUT she calls him brother damn near obsessively and maintained that role as a 'sister' for a long time cuz it was a good proximity to which she could support him. and i think at this point its just habitual to call him that but he doesnt like it either lol. so if thats a squick of yours (something that gives you an icky feeling), then it might be best to leave it. but they're not related by any means and only spent two years together as kids, during which many of their encounters were fleeting, although impactful.
Is fmc sibling with ml ?