Just to confuse you even more:
Half-brother, half-sister could be not only единокровный брат, единокровная сестра (in case they have the same father), but also единоутробный брат, единоутробная сестра (in case they have the same mother)
So, it's better to say брат (сестра) по отцу/матери as it was said by JJ already.