Technically Ivan and Alyosha have the same mother and father. Dmitri has the same father ( Fyodor), but another mother. The next brother is considered Smerdyakov. It is not proven, but it was said that Fyodor either raped or seduced Lizaveta, a retarded mute girl. He is considered their half-brother, but no one is sure, and Fyodor does not care to even consider him his son.