I think it's going to end up ruining the immersion for me if everyone has some sibling somewhere that can automatically come take over their shop for them if they die. What if I decide to kill off a whole town? Will everyone's brothers and sisters take over? Not to mention Bethesda would have to make a whole second set of NPCs with dialogue, their own AI, etc., which I can't really see happening. I think these sibling NPCs are just going to end up being cookie-cutter versions of whoever you killed, with a little bit of extra, generic dialogue about you killing their brother.
Maybe it'll only happen for essential NPCs, or something, but it still bugs me a bit.
Kills 'immersion' for you more than killing the shopkeeper and then his wife and kids just wandering the shop aimlessly with his body lying on the floor, and having the items in the shop forever unpurchasable?
And if you kill the entire town, then no, no one's brothers or sisters will take over, because they'll all be dead too... Isn't that obvious?
As others have said, the 'sister' case is a specific case - it's just 'whichever appropriate NPC is nearby'. E.g. imagine Oblivion, there's a shop in the Imperial City with two armorers, one sells heavy, one sells light armour. In Oblivion, kill one guy, his stuff is lost to the game world forever. In Skyrim, the second guy would inherit the dead guy's inventory. And not only that, he'll comment on it in dialogue.
I fail to see any way in which this is anything but extremely impressive and helpful.