If Mary could be born sinless to a sinful mother then Mary didn't need to be born sinless for Jesus to be born sinless of a sinful mother.
Whatever God did for Mary and her mother for Mary to be born sinless, God could have done to Mary for Jesus to be born sinless.
If Mary's mother didn't need to be born sinless for Mary to be born sinless, then Mary didn't need to be born sinless for Jesus to be born sinless.
Honestly, if Catholics simply thought through what they believed......
It’s turtles: all the way down.