When talking about relatives the fraction refers to the amount of variation in the population. To simplify, say every 100 bases are random and all humans share

When talking about relatives the fraction refers to the amount of variation in the population. To simplify, say every 100 bases are random and all humans share the 99% that are the same. A brother and sister have 50% in common out of the 1% that varies, so they are 99.5% identical relative to whole genome or 50% relative to the variation. It depends on what population you use as a reference too. Relative to white people, two average white people are unrelated. Relative to the 'average human' they are as related as uncle/nephew. Relative to Bushmen/Pygmy/Andaman islanders, closer still. Relative to a banana, much closer than siblings (in the usual measure) though not quite identical twins