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He hadn't even realized that he was watching his brother fall in love for the very first (and most likely last) time - not until now.
Until now, he had assumed that Magnus Bane had simply enjoyed watching Alec squirm under his outrageous flirting, enjoyed seeing a Shadowhunter so much out of his depth. He had assumed, that Alec was bothered by the way the Warlock was speaking to him, wiggling his eyebrows, winking, words always full of inuendo only when speaking to him -
He had briefly considered stepping in, telling the Warlock to get lost and not to bother his brother like that. But then again, Alec was his own man and more than capable of telling people to piss off (his mood went a bit sour at the thought of how frequently he had done just that with Clary), so he didn't bother.
And then their relationship just went tenser and tenser when Alec just wouldn't understand how important it was to help Clary, important to the entire Shadow World, but also important to this girl who he felt drawn to like he had never been to anybody else before in his entire life. Clary with her bright smile, the way she crunched her nose when she laughed, the way her hair framed her face, almost like a halo and how her slim frame seemed to fit so perfectly into his arms, like she was made to be just there, close to him, forever...
So he hadn't paid attention to his brother.
Hadn't paid attention to what he saw now, plain as day, right in front of him.
Magnus Bane had sunken into himself after he had finished healing Luke before all but falling right against Alec's chest, his head sinking right into the place between his shoulder and his collarbone. And Alec.... Alec had his free arm wrapped around him already, his other hand still clinging to the warlock's hand, almost like....
Almost like Magnus was supposed to be there, just the way Clary was supposed to be in his, Jace’s, arms.
And the way Alec was looking down at Magnus, right into his eyes?
No, Jace knew that he had never seen his brother look at anyone that way. Full of wonder, almost awe, like he wanted to stop time and live in this moment forever. Like Jace had been looking at Clary, he assumed.
"Are you okay?", he heard his brother ask and since when did Alec speak so soft, so gently to people who weren't his immediate family? Like speaking too loudly might shatter something precious, something he wouldn't get back, something he was unconsciously desperate to preserve and to protect?
There was a care in Alec's voice, the way he was holding Magnus Bane, that smacked Jace with a realization he probably would have had earlier if he hadn't been so distracted with his own feelings for a certain feisty redhead.
Magnus Banes flirting hadn't been unwelcomed. It hadn't been unwelcomed at all.
It had simply been unexpected.
But now, looking back, it was clear that his brother had enjoyed the attention the Warlock had been giving to him and to him alone. Now Jace remembered that almost bashful smile on Alec's face when Magnus had called him pretty and internally called himself an idiot.
He would have to make sure to be a better brother, a better friend, from now on.
When he saw how gentle Alec helped Magnus up onto his feet to guide him towards one of the other couches to sit down, he just felt his instinct confirmed. He was watching his brother fall in love.
His eyes went to Magnus Bane, who was still leaning heavily on Alec and something uncomfortable settled inside of his stomach. He had heard all of the stories about this Warlock. A lothario, a player, a heartbreaker.
He would need to keep an eye on things, make sure that his Parabatai wouldn't get hurt by an immortal being who was probably too far removed from this world to think too hard about how many hearts he broke, as long as he was having fun. Especially if it was the heart of a young, inexperienced Shadowhunter like Alec. Because there was just no way that a Warlock could fall in love with one of them. They probably all looked like dumb children in his eyes.
Jace just hoped that Magnus Bane would be kind enough to be gentle about it when he would break Alec's heart eventually.
He just knew one thing for certain: whatever would or wouldn't happen here in the future, him and Izzy would be there, right by Alec's side, to help him through whatever there was to come.