Respectfully, (1) what do you think the Immaculate Conception is, and (2) why would Mary having other children in addition to Jesus negate the hypothesis of an Immaculate Conception?
In the Constitution Ineffabilis Deus of 8 December, 1854, Pius IX pronounced and defined that the Blessed Virgin Mary "in the first instance of her conception, by a singular privilege and grace granted by God, in view of the merits of Jesus Christ, the Saviour of the human race, was preserved exempt from all stain of original sin."
So if Mary was no sinless and exempt from original sin, would not having sexual intercourse after giving birth of the Lord, be a contradiction ?