Years ago my parish priest gave this wonderful analogy in an attempt to explain the tension between Mary being Immaculately Conceived, and therefore never sinning, and free will.
He presented the analogy this way:
Imagine a woman living in a cabin in the woods on a hill. From her elevated position she can see the origin of a river. A town is dumping sewage into the river. Downstream there is a family living near the river. From their position they cannot see that a town is dumping sewage into the river, so they drink from that river.
However, the woman, from her vantage point, can see that the river is polluted–and while she certainly has the free will to drink from the river–has no desire to do so.
That, I think, is a wonderful way to portray the fact that there is no contradiction between having free will and never sinning.