And why are they heirs? Not because they are natural children or spirit children of God; it is because they have been adopted.
Keep reading in Romans 8 - ROM 8:15 For ye have not received the spirit of bondage again to fear; but ye have received the Spirit of adoption, whereby we cry, Abba, Father.
Or more clearly here:
EPH 1:5 Having predestinated us unto the adoption of children by Jesus Christ to himself, according to the good pleasure of his will.
and here:
GAL 4:4 - 5 But when the fullness of the time was come, God sent forth his Son, made of a woman, made under the law, To redeem them that were under the law, that we might receive the adoption of sons.
I am not disputing adoption but join heirs has to be those who will receive an eternal body of Flesh and Bone.
In the corporal state of flesh and blood would need to be abdopted in order to receive!