It seems one verse, Romans 9:11 in particular, is being used to illustrate babies even before they are born or even after birth do not sin. This is not what this verse means. What this is saying that before Rebecca's twins were born, who's father was Issac the seed of Abraham who also had the inheritance of Gods covenant he made with Abraham in Genesis 12, and before either one had a chance to do good or evil even they were predestined by God for His purpose. This verse has nothing to do with a baby from the womb sinning or not as sin is indwelt in us in the same sense like our DNA. Yes we are all wonderfully made as even in the image of God, but yet indwelled with that certain type of DNA that we also inherited by the fall of Adam and Eve.
Romans 9:10 Not only that, but Rebekah’s children were conceived at the same time by our father Isaac. 11 Yet, before the twins were born or had done anything good or bad—in order that God’s purpose in election might stand: 12 not by works but by him who calls—she was told, “The older will serve the younger.
We see here that an election was made well before either were born that the older would serve the younger and it was not based on anything the unborn child had done, or failed to do. If you look at the life of Jacob, he wasn't what I'd call an upstanding principled young man and even his name bore this out, as did his birth. Too often we believe that God chooses people based on their good moral character, but we see otherwise in the case of Jacob and Esau. Other times we want to believe that God's mercy will be never ending, but the truth of the matter is that like Pharaoh, we will all come under judgment and Pharaoh is an example of that as well.
Humanity has always had the propensity to sin and that started at creation. I would not say that God created Adam and Eve sinful, but he certainly created them with the propensity to sin. And what did God say when he had created humanity? It is good. God didn't say "It is perfect" and I think there is a big difference there.
So lets back up and look at the verses.
11 Yet, before the twins were born or had done anything good or bad.
Is one sinning by doing nothing?
James 1:14 but each person is tempted when they are dragged away by their own evil desire and enticed. 15 Then, after desire has conceived, it gives birth to sin; and sin, when it is full-grown, gives birth to death.
What evil desire would a child have? Kicking it's mother in the rib because he / she's mad at mom for eating something horrible? What then would be such an evil desire that it would entice a child to kick mom in the womb that would case a child to sin? How about when baby is hungry or needs diaper changed? Is it an evil desire to be hungry?
Just give me one example of something bad an infant in the womb would desire to do against mom that whatever baby did, would be called sin.