But I obviously share more than half my genes with my sister because my parents are not unrelated.

You're also fairly closely related to a carrot too, for what it is worth. At least as compared to a totally random string of DNA or something that doesn't even store its genotype with amino acids.

There is an impressive name for 'the amount of the novelty of the genetic heritage that can be particularly associated with you' measure but unfortunately it has slipped my mind again.