Their parents and Max had come for a visit just for the day, and Isabelle came to Magnus’ place looking for Alec and Jace, who weren’t answering their phones. Knocking didn’t get a reply, but the door was open (although doubtlessly magically warded), which was how she ended up following the sound of creaking wood to Magnus’ bedroom.
Swallowing a gasp, her eyes widened as she took in the scene in front of her: Jace sucking off Magnus while Alec fucked him with abandon. Blushing, Isabelle beat a hasty retreat. Their parents would just have to come back another time.