Another important facet here is that if you go with the theory that the curse or whatever has been causing the accidental deaths around her has an emotional trigger (whether negative in general or aimed at Tokiko), this is probably going to make Aoya vulnerable. Up until now he's been fairly detached from the whole business, it's a regrettable necessity from his perspective, but for all that he planned far ahead it still wasn't personal.
Now though? Even if he's just going through the motions with replacement little sister (which I doubt, whether or not he was going to kill her too at some point) she just stepped right onto what ought to be a major trauma button of his. I don't think he will be maintaining his calm.
