Only those who achieve the beatific vision are incapable of sin. God desires that the beatific vision be earned, therefore Adam and Eve had to be created with the potential to sin. This also applied to the angels. It was only those angels who affirmed their loyalty to God who were granted the beatific vision. Which is why there can never be another angelic fall. Or why those in Heaven will never displease God because their wills are eternally fixed to God.
Sin is a violation of the laws of God. Original sin is the deprivation of original justice, without which we are subject to a propensity to sin. Sin in any case is not a physical reality as such, although it can have physical consequences. E.g. venereal disease, obesity, bad health or even death caused by drug use...