As Mary was the mother of Jesus, she was also, like all of us, a descendant of Adam and Eve. Even though Mary was a virgin, she was in human flesh as Adam and Eve and of sin nature. How can it be said Jesus was without sin after having come into the world by human birth means from the blood of Mary?
I don't mean to be negative; I am a Christian and a believer, but as a child of God who is my Father in heaven, I naturally want to know more about my parents. The more we ask, the more we can walk in stronger faith! Being a Christian is like being a teabag, you're no good unless you're in hot water.
Let's start with a basic premise that ought to be something you would assume all Christians accept: "Sanctify them by Your truth. Your word is truth" (John 17:17). If you and I accept that statement, then when I run to something that doesn't make sense, my first reaction should be to look at myself to see if something I've always assumed to be true really does, in fact, match what God has taught.
You are assuming, perhaps because you heard it all your life, that sin or a sinful nature is inherited from Adam. Yet we can read, "The soul who sins shall die. The son shall not bear the guilt of the father, nor the father bear the guilt of the son. The righteousness of the righteous shall be upon himself, and the wickedness of the wicked shall be upon himself" (Ezekiel 18:20). Again this has been addressed before, so see "What proof is there that Mary was born without sin?", "Are Babies Born Sinners?" and "Is Sin Inherited from Adam?"