I dunno, though, I almost want to discount that kind of intrafamilial bond; after all, Slytherins are a pretty tightknit bunch, too, going by the most prominent examples we have in the HP books. Loners like Snape and Voldemort are the exception rather than the rule: in general, you've got these claustrophobic old families who serve their own bloodline first and foremost-- which is pretty much in line with Mark's approach to his relations, I think, once he gets over the instinct to assassinate them.