The major theory of Cawti's parentage comes from statements made by Steven Brust, outside of any of the published works, that Cawti is the illegitimate daughter of Prince Miklós of Fenario and his lover, Brigitta.
The source for this explanation can be found here.
There is also a hint towards this, in Brokedown Palace. At the end, when Miklós asks Devera about the fate of Brigitta, and of their child, Devera tells him: "Your daughter will be fine, Mister Miklós. I promise." [...] "I won't have to watch over her; everyone else will have to watch out for her." These statements provide an oblique hint that the daughter will be formidable, consistent with Cawti's skill and reputation as an assassin, and her determination as a force for social change.
- See also Speculation:Brigitta.