2(1)Two people are within prohibited degrees of relationship if one of them falls within the list below in relation to the other, unless—E+W
(a)both of them have reached 21 at the time when they register as civil partners of each other, and
(b)the younger has not at any time before reaching 18 been a child of the family in relation to the other.
Child of former civil partner
Child of former spouse
Former civil partner of grandparent
Former civil partner of parent
Former spouse of grandparent
Former spouse of parent
Grandchild of former civil partner
Grandchild of former spouse
(2)“Child of the family”, in relation to another person, means a person who—
(a)has lived in the same household as that other person, and
(b)has been treated by that other person as a child of his family.