This is a fairly short story. I've always been fascinated with the father/son relationship between Alec and Robert, and there aren't many fanfictions written about that. So here's my contribution :P
Notes:
There is a reason that Robert never refers to Alec by his nickname, and only by his full name. I think that a) Robert is very formal in that respect and b) he doesn't know Alec well enough to use his nickname. It's sort of supposed to show the contrast from when Magnus calls him 'Alexander' versus when his father uses it.
If you have any other ideas for me, feel free to share :)
Standing in the doorway of the Institute's library, Robert Lightwood watched his eldest son. He sat casually, his feet propped up on the arm of the couch opposite him, a book open in his palm. A slight smile played around the corner of his mouth, as if he found something strangely funny.
Robert hadn't seen Alexander act so comfortable since he was little. Alexander had been always been on the quieter, reserved side, even as a kid. Then Jace had arrived, and the oldest of the Lightwood children had withdrawn even more than before. Maryse had been concerned for a while; Alexander wouldn't speak to the two of them unless prompted to. They saw him laughing with his new parabatai and his sister, but that was it. The older their son got, the less and less he had to say to his parents.
It was only now, only after Alexander had made a scene in the Accords Hall, that it made any sort of sense. Robert's fingers tightened around the stem of his wine glass. He didn't understand how his son could have turned out so differently than the rest of his family.
"Alexander," Robert took a step into the vast library.
Startled, Alexander dropped the book he was reading and stood up. His posture was much different now; his back was straight and his shoulders were tense. His jaw was clenched, and whatever relaxation he had experienced while reading was gone. He had closed himself off. "Dad. Is there anything you need?"
"I was coming up to do some paperwork for the Clave. All of my travels have left me behind on it."
The tension in the room was almost plausible.
"If you've got to say something, just say it," Alexander sounded exhausted. "Get it over with. I already did this with Mom."
Though he was expecting it, Robert was still shocked. Was it the lack of tact in his son's voice, or just the acknowledgment that there was something seriously wrong going on?
"Where did your mother and I go wrong?" The question hung heavy in the air. Robert saw Alexander's hand tremble. It was barely noticeable. He continued, "We treated you and Isabelle the same. Jace and Max, too. They came out alright. When was it that you strayed from the pack?"
"I was eleven."
Robert took a sip of wine, and walked over to the desk. That would about the time that Jace came to live that the Institute. "You were a child. Why choose to believe in such a ridiculous notion?"
"I didn't choose this, Dad. Why the hell would I choose this? There's nothing voluntary about it."
"You certainly didn't have any regrets a few weeks ago."
Alexander was quiet, his face pale. "No, I didn't."
"What do you mean–"
"I mean that I've accepted that I'm gay, so why can't you?"
Robert flinched.
"What, are you scared by that? Upset by it? It's just a word," Alexander had curled his hands into fists, stretching the silvery scars on his knuckles. There was strain in his voice. "It doesn't make me a monster."
"It's disgusting, Alexander. It's repulsive," Robert spat, and set the glass of wine on the desk behind him. He began pacing. "Men don't fall in love with other men. It's unnatural. It isn't done! I won't have my son running around with some half-human–"
"Half-human?" he repeated slowly, incredously. His blue eyes were furious. "That's bullshit. I've met vampires with more compassion than you–"
"You don't talk to your father that way, Alexander."
"–besides, if I'm going to die young anyway, I might as well enjoy the time I've got!"
"The family name will die out. Is that really what you want?" Robert stared his son down. The way he stood was angry, the way someone looked before they took a swing at you. There was a look in his eyes too, something new: defiance. It was unnerving to Robert.
"Is that really all you care about? The name, the reputation? By the Angel, why did I ever consider doing anything that makes me happy?" he said sarcastically.
Since when was Alexander snarky? What had gotten into his son lately? "I have ambitions, and having a queer son and a renegade daughter does not–"
Alexander laughed drily. "This is about becoming the Inquisitor, too?"
He stiffened. "This discussion is over. I do not want you to continue seeing that warlock, Alexander."
"I'm eighteen. I'm within my rights to do as I like." Though he sounded confident, the color drained from his face.
"Whatever you may think, I am a Shadowhunter first, and a father second. I take being the head of this Institute very seriously."
"You're despicable."
Isabelle didn't mean to be eavesdropping. It just kind of happened.
She had heard Alec say something about 'getting it over with' and decided to stick around. Slowly, her brother's and father's voices had risen. Now her father was shouting abuse at Alec, his deep voice echoing in the hallway where she stood, crouched.
Alec was doing pretty well in there, from what she could tell. But he had to crumble at some point. It was one thing to be scolded; it was entirely different to be told that you were a disappointment. Especially if you cared as much about your family as Alec did.
The yelling reached a crescendo, and it sounded like someone was banging on the desk. Alec came striding out of the library, his cheeks flushed, his breathing shallow. He made it halfway down the corridor by the time Izzy caught up to him, grabbing his wrist. His hands were shaking.
Oh, Alec, she thought. Looking up, she tried to make eye contact with her brother, but he was staring at some point above Isabelle's shoulder.
This was his burden to carry. Izzy couldn't help; she wasn't sure that anyone could. She squeezed Alec's hand gently, and then turned and walked away. Once she was in the privacy of her own room, she flipped open her phone and dialed a number.
"Hello, Magnus? Yes, this is Isabelle… Look, can you stop by? Alec's in a bad place right now…"
Hope you enjoyed. I liked writing this, and would love to hear what you thought about how I did with the characters. I tried to read into certain lines in TMI for this, and to come up with little details.