"I wanna dress you up in silk taffeta. Tailor these clothes to fit your guilt... what's your size?"

Song lyrics Donatella by Lady Gaga.

-Anoni

PS: If you’ve seen this before it’s because I had mentioned the idea to @heronchildlove before on anon

So, Will and Tessa made sure that James and Lucie saw Jem as Uncle Jem instead of as a Silent Brother, but it had the side effect of them really viewing him as their third parent. A label Jem pretended to know nothing about. And this carried on with Matthew, because he spent most of his time with James and the Herondale’s in general.

Well Will and Tessa had been having some fun, trying to decide who was the favorite parent. Well, we know that Lucie is a daddy’s girl and that James adores Tessa, but they had been hearing this for almost a week, so they found a way to make the argument stop, and when Matthew heard about it, he absolutely had to join in.

When they actually out and out asked James and Lucie who their favorite parent was, Lucie said Tessa, Will looked incredibly betrayed and was very dramatic when he said that “Surely James won’t betray his father like this.” And James just looks him dead in the eye and says that Jem is his favorite. Matthew was very quick to mention that Will was his favorite though.

Will and Tessa knew that they had been had, but were very touched that James and Lucie had considered Jem to be one of their parents. It was all they wanted.

This can be interprete as platonic or romantic Heronchild. I find that the love between parabatai is similar to a queer platonic partnership. My interpretation of Matthew's letter to James. 

AO3: Link

Summary:

Matthew has written a farewell letter to Jamie, his parabatai, so he can work on himself.

My Dearest James,

By the time you receive this letter I will have boarded the train at King Cross to the coast. I do not mean to alarm you but I have many things to reconsider... I find myself as intolerable as a piece of lint on my jacket front. You are keenly aware of how fond I am of my jackets! After many confrontations with my mother I cannot bear to look her in the eye. I have committed such an atrocity towards her and father that they need not worry about a son who keeps up a lifestyle of debauchery. I know you have questions for me, and I want nothing more than to tell you it all. I have confided in Cordelia, and by extension I feel as if I confide in you. This is not the case by any means, I have been far too dishonest with myself, and of course, you my parabatai.