Alec and Jace are related :D

[Olivia]

As we know, there was a large family tree on the inside of the jacket of Clockwork Princess. Will and Cecily both marry and have families. Will creates the Herondale generation (Jace), and Cecily gets married to Gabriel Lightwood and continues the Lightwood generation (Alec,Isabelle,Max). SO, obviously they are related since Will and Cecily are siblings, but I don't know what you would call them. Really distant cousins? I don't know, but I do know that it is pretty awesome ( in my mind ) that Alec and Jace are parabatai AND are related. I wonder if that makes their bond stronger ....
I was just very intrigued by this information and wanted to know what you guys thought :) And of course that means Izzy and Max are also related to Jace and I think it is really sweet to know that Jace does have living extending family and they happen to be people he has luckily grown to love.

[Tharayn]

They are seven generations apart (= the parents of Will & Cecily). They have common ancestors, but I believe they aren't really related anymore.

[Nurlely]

I think those living shadowhunters do not have any family tree of all shadowhunters in general or even their own heritage.

Should Robert and Maryse Lightwoods had known that, they would be please to have one of the 'descendant' back into their wings.

But they did not take Jace based on love or compassion. They were punished by the Clave for joining the Circle, and Jace was sent to them to raise after his 'dad' died.

Aren't we all bro-sis, by Adam?