There is only one family of God…and every human being is in it.
When Jesus died on the cross, it was for the purpose of bringing humanity back to Himself. When He was raised from the dead and reinstalled as King of heaven and earth. From there He now reigns over what the Bible calls the Kingdom of God which is now the eternal order of the universe. Every person – regardless of religion – belongs to the family of God through Jesus Christ.
Not just for those who call themselves Christians did He suffer and die. He redeemed us all. You have heard it said – and it is true – that Jesus said He was the only way to the Father. He thus is the means by which we all have our access to heaven. We do not have to do anything to earn or gain this access. We are free to thank Him and rejoice in the great eternal kindness He has shown us.
I do not have to learn anything at all about a human being to know that he is my brother. Neither religion, nor race, nor nationality, nor age, nor gender, nor any other personal characteristic alters our fraternal relationship. Even if he is being disobedient to God, he is still my brother. Even if he disagrees with everything I believe, he is still my brother. Even if he says he is not my brother, he is still my brother. Therefore, I will always think of him and treat him as my brother.
Jesus did not die on the cross to create a special subset of humans to be called the family of God. He died that excruciating death to make us all His sons.