In Fire and Blood we see that Lord Rogar Baratheon's younger brother Borys Baratheon joined hands with the Vulture King and starts raiding his own people from the Dornish Red Mountains. For those crimes and to secure the Stormlands, Lord Rogar Baratheon sought his step-son King Jaehaerys I's leave to take an army into the Red Mountains to deal with the threat. Jaehaerys not only granted permission, but joined the Stormlander expedition himself. Then we see something interesting happen when Lord Rogar came face to face with his brother.
Lord Rogar’s men made short work of his own, but as the brothers came face-to-face, King Jaehaerys descended from the sky. “I would not have you named a kinslayer, my lord,” His Grace told his former Hand. “The traitor is mine.”
Ser Borys laughed to hear it. “Rather name me a kingslayer than him a kinslayer!” he shouted, as he rushed the king. But Jaehaerys had Blackfyre in hand, and he had not forgotten the lessons he had learned in the yard on Dragonstone. Borys Baratheon died at the king’s feet, from a cut to his neck that near took his head off.
Fire and Blood: Volume 1
So Jaehaerys kills Borys himself so that men could not name Lord Rogar a Kinslayer. Yet Borys was kin to Jaehaerys himself as well. King Aegon I Targaryen and Lord Orys Baratheon were half brothers. So that would make Aegon's grandson Jaehaerys and Orys' grandson Borys second cousins. While Lord Aerion never acknowledged Orys, Jaehaerys considered it a fact.
“Dragonseeds, they call them,” Jaehaerys said with obvious reluctance. “It is not a thing to boast of, but it has happened, mayhaps more often than we would care to admit. Such children are cherished, though. Orys Baratheon himself was a dragonseed, a bastard brother to our grandsire. Whether he was conceived of a first night I cannot say, but Lord Aerion was his father, that was well-known. Gifts were given…”
Fire and Blood: Volume 1
Also worth noting that Borys himself thinks that killing Jaehaerys would make him a Kingslayer, not kinslayer.
Anyway, Of course Jaehaerys was not as closely related to Borys as Lord Rogar but we know that even much distant kinship counts.
Lord Rickard Karstark dipped his head stiffly. "For that much, I thank you. But for naught else." He had dressed for death in a long black wool surcoat emblazoned with the white sunburst of his House. "The blood of the First Men flows in my veins as much as yours, boy. You would do well to remember that. I was named for your grandfather. I raised my banners against King Aerys for your father, and against King Joffrey for you. At Oxcross and the Whispering Wood and in the Battle of the Camps, I rode beside you, and I stood with Lord Eddard on the Trident. We are kin, Stark and Karstark."
"This kinship did not stop you from betraying me," Robb said. "And it will not save you now. Kneel, my lord."
Lord Rickard had spoken truly, Catelyn knew. The Karstarks traced their descent to Karlon Stark, a younger son of Winterfell who had put down a rebel lord a thousand years ago, and been granted lands for his valor. The castle he built had been named Karl's Hold, but that soon became Karhold, and over the centuries the Karhold Starks had become Karstarks.
"Old gods or new, it makes no matter," Lord Rickard told her son, "no man is so accursed as the kinslayer."
A Storm of Swords - Catelyn III
Jaehaerys could have ordered any man at arms to slay Borys and saved both Rogar and himself from the sin of Kinslaying. Or was he just counting on the fact that since Orys was never acknowledged by Lord Aerion or King Aegon and their status as a cadet branch of House Targaryen was never officially accepted by any noble, no one would be able to name him a Kinslayer?
The AWOIAF semi canon wiki seems to hold that Jaehaerys never knew for certain the circumstances surrounding Orys' birth so his belief doesn't make it a fact necessarily but given that we are talking about his decision, certainly his belief counts there.