You gain the ‘Kinslayer’ trait for having a successful murder plot discovered of a close relative. How close are we talking here? I can’t seem to find an answer on Reddit or the Paradox forums.
Thanks in advance for any help.
… need answer fast? 
Edit:
As per the wiki, it seems that the answer is “any member of your dynasty,” presumably including characters who joined the dynasty through marriage or adoption. There are a few loopholes you can use to your advantage.
Yeah, but that’s not entirely correct, because you can get it for killing off your bastard sons or brothers, who aren’t of your dynasty.
I believe they are considered members of your dynasty, even though they may not take your dynastic name.