Poly means polyamorous relationship. The definition of that is the practice of engaging in multiple sexual relationships with the consent of all the people involved. I also like to think you can do the same with romantic relationship's.
I thought this would be better than a 'who you're in a love triangle with' because I don't really like them
Jace: You, Jace, Simon
Clary: You, Clary, Maia
Alec: You, Alec, Magnus
Magnus: You, Magnus, Isabelle
Isabelle: You, Isabelle, Clary
Simon: You, Simon, Alec
Maia: You, Maia, Jordan
Jordan: You, Jordan, Jace
Camille: You, Camille, Jem
Raphael: You, Raphael, Tessa
Tessa: You, Tessa, Will (Past)
Jem: You, Jem, Raphael (Present time)
Will: You, Will, Camille (Past)