Inspired by a song by Bright Eyes called 'I Must Belong Somewhere.'
She figures everyone has to belong to someone, or somewhere. Isn't it some kind of evolutionary thing? Everything with a heartbeat has been mating since the beginning of time, so even their cavemen ancestors must have felt some pull to the others, right? Even if it was just physical, there had to have been some kind of pull.
That pull is still around today. She can see it in the way Jace holds Clary possessively when their sitting on the couch, how her mother caves into her father whenever memories come unbidden, can even see it in the way Church parades around the Institute as if he's the real one who owns it (because who says you need to belong to another? You can belong to a place, can't you?). Everyone feels that pull, bringing them together with the other person.
She wonders where that leaves her. Because Isabelle has always belonged to her brother, but even as she kisses him, she tastes Magnus on his lips.