I'm sorry but I personally feel that the SI story can work with twi'leks.

Scenario 1: your ancestor could be a twi'lek with his lekku cut off. Other examples have been known to survive this, and it sounds like an injury a Sith could have.

Scenario 2: There is in-universe evidence to say that humans and twi'leks can interbreed. You could have a distant human ancestor.