The only reason Clary is going to the City of Glass [which is the main setting for the third book hence the title] is to find a remedy that would wake Jocelyn. Therefore Jocelyn cant wake up [or at least it would make any sense for her to wake up] and tell her tale until way close to the end.
I actually really don't care about Jocelyn. I mean. we've heard her speak maybe 4 lines? So I dunno.... I think she only really holds importance to poeple because of the knowladge she posseses. [to be siblings.... or not to be]
But I really don't give a flying crap whether they are related or not. So it is a moot point all around.
Jess.... I think you are my hero. XD. lol..... Bitch slap Valentine..... XDDDD