I dont know why i cant reply to scarlet. But i think you're wrong.
The mother doesnt come from garth. The grandfather does not have red eyes hence its not a symbol of garth. Its only the blonde hair which the mother doesnt have. Their eyes are a sign that they run in the same "dirty" blood thats why the grandfather murdered the siblings to eliminate all plausible relations. They are siblings but only caleb is the true successor of garth. The rest just have the same mother which means they are step-siblings.
in the nobility world, they are considered worse than illegitimate children because its not even the same father. Hence, they were massacred.
P.S. just my theory. ? lol.