See this what Islam does to a family
This isn't peculiarly Islamic, though. It's tribal crap that predates Islam. The same thing goes on in India among the Hindus in the backwaters. And there are cases of Middle Eastern Christians doing it as well. Of course, Islam does nothing to ameliorate it.
We're gonna' go through a lot of ammo before this thing is over...
I see no reason to let these guys off the hook of "personal responsibility" by blaming it on something a part-time, barely literate Imam told them.
In fact, let's go a little bit futher ~ these two fellows enjoyed killing their sister, and would do it again, and wish they had other sisters to kill. Odds are good they used her for sex when she was a little girl.
This is a serious social problem for Pakistan to resolve ~ hanging large numbers of these pukes would seem to be a wise first step.
And inbreeding.