There is a big difference between not having original sin and not having freedom.
Not having original sin, or more correctly stated by the Catechism as having original justice, means that our first parents’ desires and action were perfectly aligned to their will; so there was no division within them. What their will recognized as good, they did without internal conflict.
As well, original justice also protected their bodies from suffering and from mortality, and gave them perfect understanding of the created world.
In contrast, we say we are prone to sin not because we cannot help but sin, but because interiorly we are already divided: what I know is good, and what my heart desire and what my flesh longs for are all different. We are weak and broken and compromised on the outset, leaving ourselves open with lots of gap for us to be tempted and assailed. As such, if we are not vigilant, we could be drawn by our flesh and committed sins of the flesh, or by our poisoned intellect and commit sins of consent, or by our devious heart to commit sins of the heart, or worst of all, by deliberate malice to commit sins of the will.
Freedom, on the other hand, however, means that we can always choose to say no, with full consequences.
Our first parents did not have original sin. They were immaculate when they were first created. There was no division within them. They could understand God’s words perfectly. They knew well God’s words and promises, and that God cared for them and made everything for them.
But they could still choose to wait on the Lord’s promise, or choose to reject Him. And reject they did choose.
In other words, having original justice protected our first parents from being drawn away by their flesh or their mind or their heart, but their will was free. The original sin they committed was a sin of the will, out of full knowledge, freedom and malice. They were not tricked by the serpent. They were lied to, but they knew they were lied to. But freely did they choose to rebel. It was indeed grave, and it is for this reason why their consequences were so severe.
Our Lady as well was Immaculate. She also always had the ability to say no to God. Our Lady is sinless not only because she was immaculately conceived, but also because she chose to follow God at every step.
In other words, being without original sin does not guarantee not sinning later on; it only means that the person was born with the grace of internal integrity of mind, heart and desire.
In Our Lady’s case, there is also a significant difference. While she was conceived without sin and did indeed consciously freely made every decision to follow God’s Will, when she gave her fiat to be the mother of Jesus, the Holy Spirit entered her in a way no other human person was privileged to receive: so much so that she was in essence personally espoused to the Holy Spirit in order to conceive God within her flesh. You can say she became Mrs. Holy Spirit. This union is so utterly sublime that it would be impossible to even think how anyone possessing such deep union could ever imagine sinning. This is not out of lack of freedom, but fullness of love.
And from this we also can understand why the saints in heaven will never sin. They are in fact, shockingly, fully capable of sinning, for they are utterly free. But all in heaven who are possessing the beaitific vision of God are so utterly fulfilled in love that sinning would become utterly foolish and undesirable.