Excuse me if I am mistaken, but Alec is not just Jace’s friend or foster brother. This is Jace’s paratabi that they are freaking talking about. You would think that if anyone would know if Jace was lying it would be the one person that is you know BONDED TO FUCKING HIS SOUL! Again, why did Clare write the whole paratabi thing into her books in the first place? To me, it means very little to anyone including Clare herself. If Clare doesn’t show that being paratabi actually means anything, then it becomes little more than hyperbole.
Although, I have a lot of issues with the television adaption ‘Shadowhunters’ right now. The story arch between Alec and Jace on the show really makes it clear that Clare missed a golden opportunity by not giving this piece world building the attention that it deserved.