She;s also half demon, which she got from her dad, was born in the Entertainment District, and her blood can turn any flower into a blue spiderlily, which put her at risk as a child because Muzan Jackson wanted the blue spiderlily.
I turned you and your brother into Hashiras.
Same breathing style, different, as I like to say, postures. It won't work properly with one posture on its own. So it needs both.
Tied together with both of y'all having demon forms that are upper moons and Yume being the first to get the demon form with a personality change for noot, and boom, those 2