Diodotus isn't really a relative of Seleucus, just a political successor in the Baktrian region. You don't need to be a relative to be a successor. The five good Emperors of Rome were all non-related, and appointed by their predecessors.

well. i was thinking that the fact that Baktria fought for freedom, and it was later ruled by someone who was not apointed by a successor/successor descendent.
After Perdikkas, none of the Successors were appointed. Perdikkas was the regent after Alexander's death, but he was murdered by Seleucus and a few others when his attempt on Ptolemaic Egypt failed miserably. Seleucus was the satrap of Babylon, loyal to Antigonos before he felt threatened, and escaped to Egypt. He fought alongside Ptolemy, before returning to Babylon with a 1,000 soldiers as a gift from Ptolemy, where he reasserted his rule. He would later defeat Antigonos and Lysimachus to extend his rule, but these regions would be reasserted under new successors after Selecus' murder.

As you can see, the legitimate successors all asserted themselves in less then epacable fashions.