Sorry. This is more fangirly than anything but I have get it out there. Any of you guys read ‘The Infernal Devices’ and ‘The Mortal Instruments’? I have a feel-killing theory…
‘”Remember Will Herondale? I know you do. That boy of yours, that Lightwood. THEY EVEN LOOK ALIKE”’ ~ City of Fallen Angels, page 271
Will and Alec supposedly look alike. Cecily and Gabriel (so far) haven’t been paired up with anyone. What if Gabriel started courting her to piss off Will and they eventually fell in love and got married? Then their direct male descendants would have the last name ‘Lightwood’ and they might have inherited the appearance of Cecily, making them look like Will. That would explain the resemblance between Alec and Will… O_O
My feels are now dead and gone. It all makes too much sense.
Saying that, I’m assuming Tessa wasn’t barren and ended up with Will after Jem died (I’m suggesting this even if I don’t ship Wessa). They had children and Jace is descended from them. But then how I hear you ask, did Emma Carstairs cone into being? Tessa and Jem DID get married but Jem died when she was pregnant with a son. THEN after she gave birth, she decided the baby needed a father figure in its life, so she pursued her relationship with Will who still loved her. The baby kept his father’s last name and kept the Carstairs family bloodline and passed it down to Emma… O_O
‘”Do they?” Magnus said, as if he had never thought about it.’ ~ City of Fallen Angels, page 271
The ‘as if’ implies that he could’ve actually known why they look alike. Magnus probably would’ve known about all of this when he was in London in 1878. When he met up with Tessa in the epilogue of ‘City of Glass’ he could’ve asked Tessa about Alec. Then, in ‘City of Ashes’ when he secretly starts dating Alec Lightwood, he could’ve already known.

Thoughts?
~ I sincerley apologize for killing your feels.