A person is a member of another's family within the meaning of this Part if—
he is the spouse or civil partner of that person, or he and that person live together as if they were a married couple or civil partners, or
he is that person's parent, grandparent, child, grand-child, brother, sister, uncle, aunt, nephew or niece.
For the purpose of subsection (1)(b)—
a relationship by marriage or civil partnership shall be treated as a relationship by blood,
a relationship of the half-blood shall be treated as a relationship of the whole blood,
the stepchild of a person shall be treated as his child, and
an illegitimate child shall be treated as the legitimate child of his mother and reputed father.