I am only killing three brothers because I only want loot from those brothers. Each item has a drop rate of 1/384, so each brother, having four items total, amounts to a chance of 4/384 -> 1/96. This is additive, so if I kill three brothers, I have a 12/384 -> 3/96 -> 1/32 chance of receiving any of their items from a given chance. It is a 1/16 chance of receiving an item if all six brothers are killed.

tl;dr regardless of how many brothers you kill and what your KC is upon looting the chest, you still have the same chance of getting a particular item.