Things we know about the lost Herondale timeline
Spoilers if you haven’t read The Wicked Ones
1828: (The Lost Herondale) Catarina saved Tobias Herondale’s son (which will become the lost Herondale line)
1936: (Learn About Loss) One of the descendants of Tobias Herondale’s son became involved with high Fae and produced descendants with fae lineage and whoever that fae was, she was very important (enough to have the fae searching for the lost Herondales)
1940: (A Deeper Love) Basically proof that the faerie woman was important
1989: The line continues down to Rosemary Herondale*
~1997: Kit was born (and by Lady Midnight he would be 15)
*Rosemary was described as “younger than Crow” in the Wicked Ones and Crow looked “barely into his twenties”. But they also know how to change their appearances. So how old exactly is Rosemary Herondale in 1989? Because the time gap (which is at most ~11-12 years) sure is not large enough for Rosemary to be a grandmother if she was really as young as she looked. So that leaves four explanations (that I can think of):
1) Rosemary was much older than she looked which means she could be old enough to be Johnny Rook’s mother. Kit’s mother’s identity was still unresolved.
2) Rosemary was as young as she looked and Johnny could be her sibling or something (I mean, it’s not like it’s impossible to have two children). Again, Kit’s mother’s identity was still unresolved.
3) Rosemary was as young as she looked, and she was actually Kit’s mother but that would leave Johnny’s Herondale lineage unexplained (Or even Shadowhunter lineage, because it’s not the first time the Shadowhunters fail at keeping family records straight. I mean just look at Jace’s many last names throughout TMI and that hugely misleading family tree in Clockwork Princess. He could honestly be anyone)
4) A combination of Rosemary being Kit’s mother and Johnny being her sibling, but that would mean Jem and Tessa either didn’t know and just assume Johnny was his father (because he would also be a Herondale), or they deliberately let Kit believe Johnny was his father when Johnny was his uncle or something. And then it becomes the question of who Kit’s father was...
Feel free to supply more information (or scenarios) if I missed any :)