He's very logical about things.
On one hand, the sister died years ago and he never saw it happen. On the other, his assistant was murdered in front of him.
Now that he can't bring her back he's putting her into the past as well.
I wonder if he's just overtly logical or has a form of an empathy development issue. Some people are just born without the ability to empathize with others or are able to logically put tragic events behind them very quickly. It makes them seem cruel, but it's just the way they are.
Jeez I'm so sad right now. It's not like that at all. Finish reading to the end and it vaguely makes sense.
He's more sad about that girl dying (and yeah i know he was close to her) but come on hes more sad about her than he is of his own daughter