I've noticed that, probably biassed by the "mediaeval city" in the first post, my reply holds for something like the German principalities or Italian city states, but would be considerably more complicated in a larger kingdom, like France or Spain. The aristocratic families are generally landowners, and members of the clan would be at the ancestral holdings, frequently several days travel from the capital city and royal (ducal, whatever) court.
In the absence of reliable communications, co-ordinating the massacre becomes much more of a problem; you're forced to assume your other conspirators have got their bit right. If any segment fails, it puts the whole in jeopardy. Treaties with foreign powers will probably hold as long as your legitimacy is unquestioned, but a nephew/true heir to the throne of is a powerful playing piece in international diplomacy.
In the absence of reliable communications, co-ordinating the massacre becomes much more of a problem; you're forced to assume your other conspirators have got their bit right. If any segment fails, it puts the whole in jeopardy. Treaties with foreign powers will probably hold as long as your legitimacy is unquestioned, but a nephew/true heir to the throne of is a powerful playing piece in international diplomacy.