Addressing the main topic of the thread... am I the only one who took Thirteen's line about being a sister in exactly the same way that I took Twelve's line about being a Vestal virgin? To me it was just a bit of banter about having once been/pretended to be something traditionally feminine. It's a variant on the classic Doctor "name dropping" that Four and Five excelled at, and all of Seven's befuddled ramblings, and all of the myriad references that all of them have made in Big Finish stuff. I don't think for a moment it was meant to be taken literally and that there was a previous female incarnation heretofore unknown.
Now, someone over on Divergent Universe is adamant that all this means that Chibnall is setting up for "pulling a Moffat" and introducing another "lost" Doctor. I think that's a massive stretch. It's just a throwaway line. Have we in fandom all become so precious about every little thing that we have to take what's obviously a jokey aside and turn it into some vast conspiracy?
As for the Master... I always thought that the Pratt/Beevers Master was the Delgado Master, just burned beyond recognition. I always thought the "it's his last incarnation" fact originated during the Pertwee era, long before The Deadly Assassin. And thus, the Ainley Master was just the burned Delgado Master having merged with Tremas' genetic material, or possessed his body somehow, and that he was still on the last incarnation of his original cycle up through the TV movie, having escaped execution as the "deathworm" to possess yet another body. And when Jacobi showed up in the New Series, I just assumed that was the first (or at least an early) incarnation of a new cycle granted when the Time Lords resurrected him for the War, leading to Simm, then (presumably) Missy.
I have no idea where Big Finish's Alex MacQueen Master fits into all this, as I haven't gotten that far yet. But as far as televised Who, that's what I've always thought based on what I remember from childhood PBS reruns and NuWho. What am I missing?