By Mary accepting Christ for who He was, she would be without sin because she has excepted the fulfillment of the law and salvation from Original sin. Thus Jesus wouldnot have sin becuase it is not inherited (besides being God). How would Mary avoid inheritance of sin if her parents had sin?
Or am I over thinking it somehow? or is it flawed?
Well, I think you’re as many of us when we are confronted with divine mysteries

. Your question is interesting and complicated to answer because I’m not a theologian and it’s easy to fall in pelagianism or another heresies if you say things without care. The Inmaculate Conception, as the Holy Trinity, is a sacred mystery very difficult to understand, so I’ll try to explain my perspective, the best that I can, although I’m afraid I could easily fall in some sort of heresy . Apologies in any case.
What is the original sin? In a simplified way the original sin is a true and real sin. It’s not an extrinsecal imputation of Adan’s sin but a sin that spreads by inheritance and therefore each one of us has an inherent original sin. The sin is inherited, but not theorically imputed but personally suffered. The original sin consist in the absence of divine and holy grace. And is a sin attached to the human nature because is by means of generations as we inherit the sin directly from Adam. So it’s transmitted. The sacrament of baptism washes the original sin but doesn’t remove the natural concupiscence, neither the cause of the sin (Adam’s disobedience) that is latent and vigent in any process of generation.
So, in the beginning we can understand that the biological act of conception (but not the divine act of creation of the human soul that is parallel to the act of conception) should imply the original sin because it’s the human generation the process throught the original sin is transmitted. Mary and Jesus were **biologically conceived **and therefore Jesus, not as fully God but as fully human, and Mary
were suceptible to receive the original sin by biological inheritance. But the original sin was not transmitted to them. The questions appear then: How? When? Why not only Jesus? Why both?
The reason of why the original sin was not transmitted was the preservation of the complete holiness of the second person of the Holy Trinity in the process of incarnation. The Lord Jesus is fully sinless. Jesus as fully God is in nature sinless but also as fully human,. The
homousios guarantees that His two natures (divine and human) cannot be different (for example a divine nature sinless and a human one with sin). That ‘sinlessness’ includes the absence of the original sin. Moreover Jesus is the source of all graces and the original sin means absence of divine and holy grace. If Jesus had “suffered” the original sin He couldn’t be the source of any grace. It’s a logical principe: you cannot give what you do not have.
The transmission of the original sin was ceased in a point of the generation before Jesus. The Catholic Church teachs us with its authority the point was when Mary was conceived. It’s logical to think the stop of the transmission of the original sin happened in Mary’s life in the sense that the best and more natural way to preserve Jesus ‘sinlessness’ being He conceived in His mother’s womb (sharing blood, flesh, etc) is preserving His mother (in flesh and spirit) from any sin.
Some people here make question like: In what moment was Mary preserved from sins? Is really necessary that she was preserved of the original sin? Why not only preserved by sins just after she was born? Or only just before Jesus’ conception? The reason is easy. The original sin is forgiven in each person by infusion of God’s grace
but the effect of the original sin cannot be eradicated because in the process of biological generation is inevitably present. And is inevitably transmitted So God not only had to infuse full grace to Mary but also excepcionally to eradicate the biological dimension of the original sin in her conception to remove completely the transmission. Therefore Mary cannot have original sin. A Virgin Mary with original sin necessarily imply transmission of the original sin to Jesus..
In this point one must ask: But is it possible to think Jesus’ sinlessness could have been preserved in other way?, lets say for example, in the moment of Jesus conception. It would be perfectly possible, but the original sin (as any sin) needs to be removed (washed) by means of God’s grace and Jesus’ redemption.
Jesus cannot redeem Himself because He is the redemption of the world. He doesn’t need to be redeemed. The redemption cannot be redeemed like one cause cannot be efect of its same cause (‘Causa sui’). At the same time He was conceived with His divine graces so
He didn’t need to infuse more graces to Himself to preserve Himself from original sin. Grace and redemption only can be applied to someone with necessity and conditions of receive redemption and grace. Therefore it’s not logical to think the preservation of the sinlessness of Jesus should be aplied to Him.
Although He didn’t need any grace, neither redemption, He, being biologically conceived was in risk to receive the original sin from His mother. Therefore it’s necessary His mother was preserved from original sin by God and thanks of the infinite merits of his Son.
That’s my aproximation to the subject and probably it has some theological errors. In any case, I prefer to trust in the infallible authoriy of the Church that in my fallible knowledge. Perhaps the most solid argument in favor of the Inmaculate Conception is hidden in the Sacred Scriptures and expressed by many Church Fathers. They explain better why Mary is the Ark of the New Covenant, why she has to be completely inmaculate and pure to bear inside God. That means without any stain of sin, included the original sin.