[topcat54]

I've read many times that it appears Jesus had siblings. Were they also, then, immaculately conceived?

Even in the IC theory were true, Mary's other children would not have been immaculate since they had a human (non-immaculate) father.

But it's all speculation. The Bible does not teach that Mary was immaculately conceived, and it does teach that Jesus had (step-) brothers and sisters.

[Grey Ghost II]

Mary's other children would not have been immaculate since they had a human (non-immaculate) father.

What other children?