Here are some of my theories for what'll happen in future with Emma and Julian's relationship, and specifically the consequences of a romantic parabatai bond as revealed near the end. I'm writing this before reading the next book - so who knows, maybe I'll be proved wrong soon.

My first theory is that there is no curse. Possibly the first romantic parabatai couple just became naturally corrupted, and the Clave at the time wrongly assumed it'd been caused by the bond and their warlock-like rune powers. Or possibly they didn't even go corrupt at all. Shadowhunters back then could have been so...