There is no proof that Jesus had brothers and sisters. When it mentions "brethren" in the NT, it could have meant any family relations, like cousins. To back that up, go to the OT where it mentions Abraham and Lot being "brethren". Lot was Abraham's nephew.
If Jesus had brothers and sisters, why would he "give" His mother to John while He was dying on the cross? John took Mary into his home that very day - this would not have been proper if Mary had other children that should have taken care of her.
Why is it hard to believe that Mary remained a virgin after having Jesus? Did she not conceive a human child through the Holy Spirit?
BTW - did anyone ever think about where those 46 chromosomes of Jesus came from? If Mary gave Him 23 from her egg, and there was no sperm to contribute the other 23, where did they come from?