A ton of us Boarders have been debating the taboo against kinslaying in Westeros, and have been trying to define exactly what "kinslaying" is. Is it limited only to the situation where you kill a kinsman by your own hand, or would it include anything that leads to the death of a kinsman? In other words, had Renly's army defeated Stannis', and Stannis died in the battle at the hands of Loras Tyrell, would Renly still be considered a "kinslayer"?
Maybe by some, but that's a stretch.
There are degrees in kinslaying, as in anything else. Fighting a battle in which a brother dies might be frowned upon, but killing him with your own hand would be considered far worse.
In the scenario you propose, another factor might be whether Renly gave any orders in respect to Stannis. Did he command Loras to kill him, or offer a reward to whosoever slew his brother? Did he tell his men to see that his brother was taken alive? Did he not address the issue either way? There are obvious degrees of guilt, depending...
The other factor, which you haven't raised, is degree of kinship. Killing a parent is probably worse than killing a sibling, but either one is a lot worse than killing a distant cousin. Lord Karstark was stretching that aspect of it when he tried to accuse Robb of kinslaying... but of course he was hoping to save his head.