D won't ever lay a hand on Kala, because he won't feel he has a right to. His father is the reason that her father was killed. That alone will "handicap" him in this battle against her. In addition to that, he does still consider her his older sister and the power she has over him might've ceased somewhat since Naku came into the picture, but she still has some kind of hold over him, which might prove to be a hindrance in itself.
However, I do believe that he might feel forced to do something about her if she threatens his family. I also believe that if she ever threatened Naku's life, he wouldn't hesitate to disarm her. He'd be willing to let a lot of things go, but he wouldn't tolerate the risk of losing Naku or any of his other family members at her hand. Especially Naku.
So, let's see. This is why I think that their power-struggle could turn out really interesting. On one hand, D would never openly fight her, but on the other hand (like Kala said) that when you fear losing everything you hold dear, you cross every line. I'm curious to see how far D will be willing to go in this war against his own blood.
In the end, it must come to an eye for an eye. His father killed her father. She killed his father. And now she's threatening his entire family and Naku. I believe that D can endure a lot of things, but even he has a limit. If she crosses his line, he will fight back. No restraints.
