It depends on the connection between the half sisters. Let us assume that your mother and sister have children. You and your aunties children are first cousins. Your aunt and uncle split up. Your uncle has another child with his new wife. That child is the half sibling of your cousin because they share a father but the half sibling is not technically related to you. Then your aunt marries again and has another child. That child is your cousin, and is the half sister of your cousin.