Families is more than one family (plural). Family's is when the family owns something (possession) eg. the family's dog, the families gathered
No
Family's - belongs to one family e.g. this is my family's dog Families' - belongs to many families e.g. the families' school was shut
family therapy is a type of therapy that helps bring you and your family closer together
A large family has more people where as a small family has less people.