is he gonna uding mc as vessel for his blavk magician's "sister"

I don't think so
Even if he tries it won't happen cuz obviously the story will end there then
（￣へ￣）

Doubt it. As you can read from his lines, he used to think about using her as a vessel but then he hesitated or changed his mind after 2 years of observing and staying by her side. He has grown quite attached to her, even enough to do her requests and let her investigate him to see if he is a friend or a foe.