Is agnate considered as half or whole siblings?

Children are half-siblings when they share only one biological parent.

half.

My father had an affair early in my parents' marriage. I found out at the age of 27 that I had "another sister". Now, due to the fact that we obviously have different mothers, I refer to her as my...

whole

A half brother or half sister sharing the same father is an agnate sibling. Those half siblings sharing the same mother are called uterine sibling.