How do starr and Kenya share an older brother, seven, but they are not related

In the novel The Hate U Give, Starr and Kenya both have an older brother named Seven. Starr and Kenya only know each other because of their older brother even though both girls have no relation to each other. This is because Seven's father is Starr's father, and Seven's mother is Kenya's mother which makes them both related to Seven but not directly to one another.