7 is correct.
Think of it this way:
John and Jane have a child called Steve. Steve Marries Mary. Steve and Mary have three children, Jack, Sarah and Melanie.
John is the Grandfather, father number 1 and the father in law
Jane is the Grandmother, mother number 1 and the mother in law
Steve is father number 2 Child number 1 and son number 1
Mary is mother number 2 and the daughter in law.
Jack is Child number 2 ,Grandchild number 1, son number 2 and the brother.
Sarah is Child number 3,, Grandchild number 2, sister number 1 and daughter number 1.
Melanie is child number 4, Grandchild number 3, sister number 2 and daughter number 2