I’m a Witch Who Mistook My Vampire Sister for a Power Bank

After turning into a witch, I have to absorb other people's magic to survive.

A vampire often shows up near my house. She's very weak, the perfect living battery.

I often drain her, let her go, then drain her again once her magic recovers. I keep repeating the cycle, constantly absorbing her magic.

I could have kept doing this forever, but tonight, after draining that vampire, I came home.

And I actually saw that vampire in my younger sister's room?

The vampire I've been draining is her!

But here's the question: why is she in my sister's room?

Could she be...?!

Vampire: "Big bro, let me explain!"

(Permanently female later on) (The characters have lots of secret identities)