
If she's a Rabit, they're not related by blood. So how did he identify her with his magic in from of everyone as his sister?

I guess if you looked at it another way it could be that her blood is linked as a member of the Baili family since she was registered as an adopted daughter.
So she doesn't have the same blood as the rest of them but her blood is still under the name of the Baili's
If you understand what I'm trying to say lol, doesn't make complete sense but if you really wanted to find a reason I guess it could work

I still think magic is the way to go with this one, he did say blood relation and by the looks of it they're not related by blood and signing a peace of paper would not change anything in regards to the spell he used if it was looking blood relations the only thing that could make the added to the family as blood relation would a magic so all in all
MAGIC!!!!!!!
(╯°Д °)╯╧╧
I wanted to c what suck so bad, but b4 I knew what was up, the thing ended? JAJAJAJAJA! WHAT!? LOL (ノ≧∇≦)ノ