For the purposes of this Part one person is a member of the family of another if—
they are, or live together as if they were, spouses or civil partners, or
one is the parent, grandparent, child, grandchild, brother, sister, uncle, aunt, nephew or niece of the other.
For those purposes—
a relationship by marriage or civil partnership shall be treated as a relationship by blood (and, in particular, P's stepchild shall be treated as P's child), and
a relationship by half-blood shall be treated as a relationship by whole blood.