That is your assumption. Adam and Eve had no sin nature, yet they sinned. Satan was not created with a sin nature, yet he sinned. So, this idea of yours that a person MUST have a sin nature to sin is totally refuted by scripture.
I actually agree with this, "from the womb", that is, after we are born we go astray in sin. But we are not born sinners.
Jesus himself refutes you, Jesus said if he were to deny his father, he would be a liar. His sinfulness was determined by what he DID.
Jhn 8:55 Yet ye have not known him; but I know him: and if I should say, I know him not, I shall be a liar like unto you: but I know him, and keep his saying.
Jesus said that if he said he did not know his Father, he would be a liar, he would be a sinner. Jesus being a sinner or not was determined by what he did.
Jesus implied he could sin, he said "if" he denied that he knew his Father he would be a liar. The word "if" implies that it was possible for Jesus to deny that he knew his Father and be a liar.
Why would Jesus imply he could lie if he was unable?
Jesus was not a sinner because he always obeyed God's commandments and never sinned.
jesus was BORN as the sinless lamb of God, and since he was ALWAYS God, he became a man without a sin nature!
he was defined by being "in the beginning was the Word, and the Word was with god, and was God", NOT to how he lived while on earth!
Again, jesus unique, NO other Human was born like him, nor could keep the Law as he did, not claim to be sinless, nor God Incarnate!