Clockwork Princess theory
I know there are a ton of these, but I read this theory on tumblr somewhere and added a few of my ideas to it as well. Credits to the person who spotted this.
Basically, Mortmain said that Tessa’s mother was a Shadowhunter, and that “she didn’t know what she was”. So why was that?
Well, when Will and Tessa crashed Benedict Lightwood’s party, the faerie woman came up and told her about changelings, but said that Tessa definitely wasn’t one. So why would she have said that?
Well, my guess is that Tessa’s mother was a changeling. A shadowhunter baby, swapped with a mundane child. So Mortmain could have swapped her so that Tessa could be born.
But then question was, who was she swapped with? If she was swapped, then who was the mundane child that replaced her in the Shadowhunter world? Well, the answer came in the Clockwork Princess prologue.
I mean, the Startweathers are an extremely strong, pure line of Shadowhunters and there would be no reason for Adele not to have been able to receive the marks and turn Forsaken. But she did, the way a mundane would have if given marks.
So what if that child who turned Forsaken was the changeling child? If she was traded with the real baby, who became Tessa’s mother, then she’s a mundane and logically would have been unable to receive the Marks.
And remember in Clockwork Prince when Aloysius Starkwether saw Tessa, he had a shock and recognised her, even though Tessa has never seen him before. Since being born a warlock means that you would have no physical characteristics of the demon parent, she probably strongly resembled her mother. And there’s a high chance that she resembles someone from the Starkweather family. So the recognition makes sense!!!
So that’s my theory, tell me what you think! :)