I think it depends on how your first cousin and her half sister are connected. In other words, let's assume that your first cousin and her half sister share a mother but have different fathers. Their mother is the sister of your mother. This makes both the first cousin and her half sister your half cousin. Now let's assume that your first cousin and her half sister share a father, but have different mothers. Your first cousin is your cousin because her mother is your mothers sister. But her half sister has a different mother. The half sisters are related because they share a paternal father. Their father is not related to you by blood, he is the first husband of your aunt, technically your uncle or ex uncle by marriage.
sis not related to by blood. Her mother is actually someone