I love how they delve into the "what makes one family" thing. There's the "actual" big brother but he's way too down the toxic hole because of their parents' death, and there's no adult he can trust anymore. And there's the "fake" big brother but he's really kind and treats Kanami like an actual little sister. Or maybe even better than your usual big brother, and like the picture perfect one that many dream of. (Their reasoning could also be as different as their actions though, that's just a theory.)
Personally, I think blood family is important but if they've become too toxic, and no honest communication works, it's better to just distant yourself with them. Not until they decided to change and are working on it, or they don't bring you trauma anymore, it's for the best to just slam that door. I know how much one may yearn for that connection, nevertheless you shouldn't yearn for something that just hurts you.