Since when did one person make up the family tree of both of your parents?
Aren't Twi'leks only able to have Children with other Twi'leks?
Also, here is something to consider about a Pureblood Inquisitor: they were a late addition to the game. Originally (aka a few months before launch), if you wanted to be a Pureblood you had to be a Sith Warrior. It was because all Purebloods are Force-Sensitive (those that aren't get killed as infants), and they are part of the Aristocracy. So the "Slave Discovered to be Force-Sensitive Narrative" Inquisitor backstory didn't fit with Pureblood's. It was only after a large fan outcry (on the Pre-Release SWTOR.com there were polls with thousands upon thousands of votes with around 75% of people in favor or Pureblood Inquisitors). And more importantly, a good number of Bioware Devs were in favor of allowing Pureblood Inquisitors. As a result, they were added a few months before release.