None of the PCs have ever heard of House T'sarran.
The two houses in Maerimydra that were important to House Millithor were Morcane (your trading partner) and Chumavh (the most powerful house).
You know that Maerimydrya had nine major noble houses (the first of which was Chumavh), and that T'Sarren wasn't one of them.
So either 1) House T'Sarren was so minor that nobody noticed it--i.e. no noble compound just a building on a street in the city 2) House T'Sarren is actually from another city, 3) House T'Sarran is a former noble house (i.e. thought destroyed), 4) House T'Sarran has never previously existed, or 5) You are being lied to.
With regards to lies and sense motive, Matron Ki'Willis basically believes that Dorina is attempting to tell the truth. Dorina may be twisting various things to fit her viewpoint and she might be insane, but the vast majority of what she is saying are things she believes to be true.