Q. Since Mary had other children, the brothers of Jesus, would they not be sinless also?
A. The brothers and sisters of Jesus are kinsmen of Jesus or else Joseph’s children by previous marriage. They are not full siblings of Jesus. They are either cousins or step-siblings. Mary gave birth to only one child and remained a perpetual virgin. Even Martin Luther and John Calvin attest to this fact.
Even if Mary and Joseph had other children together, they would not have been sinless since Joseph was not sinless.
Q. Why was Joseph not sinless?
A. Joseph was not preserved from the stain of original sin by the power of God—Mary was.
Q. But, Mary came from the seed of David who himself was a sinner and murderer.
A. Mary was preserved from the sin of Adam and her ancestor David by the power and miraculous intervention of God the Father at the first moment of her conception. She was free from all sin and a sin nature. Remember, both Adam and Eve were created immaculate also. They were not doomed to sin like all of us. They lost this great grace, the fullness of human perfection, when they chose to rebel against their God. So, they were unable to pass on the fullness of grace to their offspring.