Um ... what?
Most murders are crimes of passion. Which indicates the two people knew each other. I'm willing to wager many of them were family members.
Not sure exactly where you are getting that.... only 14% of murders are by family members. 30% are where the victim and killer knew each other. And the vast majority of those are acquaintances. So "yeah I met bob at a beer party last month".
44% of murders, the police have no idea what, if any connection there is between the victim and killer, and 12% they know are strangers.
Now, I'd take a guess that the police could most likely figure out if those 44% were family related.... don't you?
So I'm not sure where you get the most murders are crimes of passion.... unless that's a very loose definition of passion., and even so, most are certainly not family.
In fact, even the majority of "family" murders, are broken family murders. Estranged wife, divorced husband, step kids and so on.
Yet another reason for people to quit being so selfish, and work it out instead of divorce... but I digress.