If it was necessary for Mary to be "free from sin" so Jesus' would not be compromised, then wouldn't the same logic hold for Mary's mother? Would she not have needed to be "from from sin" so that Mary would be free from sin, if it's necessary for Mary to be free from sin to have Jesus free from sin?
And also for Mary's mother's mother...mother's mother's mother....
The Immaculate Conception is all about Jesus. A pure womb for him in the Ark of the New Covenant.