This is probably a bit AU because it doesn't really follow the tmi story a lot. I apologise now since I've only read CoB and it probably doesn't really follow the story.
Cassandra Clare owns them.
Isabelle Lightwood couldn't sleep. She lay awake in her bed, her mind wandering. She was wide awake, bored and even a little turned on. Scratch that, turned on. Like really turned on.
Her thoughts went back to what had turned her on in the first place; Clary Fray. She knew that Clary was with Jace, and that the other girl wouldn't ever think of her the same way...But Isabelle just couldn't help herself. She liked Clary. A lot. Isabelle had thought a lot about Clary over the past few weeks, and it wasn't all innocent thoughts either.
Isabelle was too frustrated to sleep. She glanced at her alarm clock. It was 3:24 a.m. She sighed, closing her eyes and trying, yet again, to go to sleep.
Her thoughts turned to Clary again. How Clary had looked in that dress on the way to Magnus Bane's party. And to what she'd look like without it...Isabelle's hand had slowly been moving down her body, inching towards her crotch. She sighed again, giving in and allowing herself to fantasise about the red head. Her hand sliding under the thin fabric of her panties.
She thought about what it would be like to kiss Clary. About what it'd be like having Clary's fingers massaging her instead. About what it'd be like to pleasure Clary, and to reduce the girl to a moaning mess. Isabelle's thoughts then went to having Clary underneath her. Then to having Clary's head between her thighs...A few minutes later Isabelle let out a small whimper as she climaxed and came.
She lay there for a few minutes, feeling slightly guilty. Yes, she did like Clary, but Clary was also one of her closest friends now...
She exhaled. Tomorrow she'd go and see Clary, and find out if she had any chance with the beautiful red head.