Board Thread:Legitimate Canon Questions/@comment-32617252-20170808034550/@comment-4091815-20170809073702

Theo and his sister were humans. Theo wanted to be special and the Dread Doctors offered him the chance to be special. He needed to become a chimera first in order for them to experiment on him.

He needed to have two different types of DNA inside his body so he let his sister die and they implanted her heart in him.

Now, with the two different sets of DNA, he was suitable for experimentation and the Dread Doctors worked on him and turned him into a Chimera Werewolf mixed with some other stuff.

Bottom line, Theo's sister was human. Now she's dead.