I thought she was just jealous but she was actually worrying about him getting his feelings hurt, so she cheered up after knowing that he might actually have a chance. Such a sweet sister.
I wonder if their urge is for the blood itself or the act of sucking them directly from the host. But they did mention something about sucking and not fitting in with society, so I guess it's a bit of both.
Lol at her dramatic visual thought process.
And I love how her reaction goes from really negative to maybe slightly positive(?) after realizing that they're siblings, is she a wincest women of culture? And a BR variant at that?!