Responses
From what I understand, the mothers of tsubaki and akou were siblings while enjo's wasn't. So even though the 3 of them share the same father, akou and tsubaki are more closely related; they're both brothers AND cousin. As for enjo, to 2 of them, he's s just a (step)brother, so their attraction's to enjo are not as strong as they're with each other.
But well correct me if I'm wrong.
There is something that I don't understand. If all three of them are brothers, why are the other two brothers not in love with each other??