"For whoever does the will of my Father in heaven is my brother and sister and mother." - Matthew 12:50
Being a member of any family comes as a result of birth or adoption. One is not expected to work or contribute funds to join up. A family is not a club, but in most cases those who are members of a family show visible evidence of their connection. Family members will often bear a physical resemblance. They will also have certain mannerisms that are shared by the rest of the family. And finally, those who are members of a healthy family will make every effort to please and honor the family head.
The above mentioned picture is exactly how the family of God works. We are born into His family by faith in His Son, Jesus Christ. This birth is a gift from God and it is eternal in it's nature. While we can never work for, or contribute funds to join the family of God, we should live worthy of the calling. Children of God are those who reflect the image of our elder Brother, Jesus, and they make every effort o do the will of their heavenly Father.
Just because we are given the gift of a heavenly family tie, doesn't mean we are free to live like other families. We are told in Romans 8:29, "For those whom he foreknew he also predestined to be conformed to the image of his Son, in order that he might be the firstborn among many brothers." The family likeness is seen in the faithful efforts to be like Jesus, by pleasing and honoring our heavenly Father in doing His will.