I think I assumed T'Pau was part of the Spock/Sarek family primarily because earlier novels did-- I probably read Sarek and Spock's World long before I ever saw "Amok Time"! Hence her inclusion in that role in The Tears of Eridanus; it never occurred to me to question the assumption until now.
You're not the only one. I've made the same assumption about T'Pau in my
Enterprise novels, and I'm pretty sure Mike Martin did in his. (By the way, I drew on
The Tears of Eridanus as a reference for Vulcan culture in
Uncertain Logic. It's an alternate culture, of course, but it has some useful stuff about the pre-Awakening era.)