The first time Isabelle kisses him, she's six and he's eight, and she tastes of chocolate and sunshine.
He only smiles when she does this because everything was much more innocent then. It was a sisterly type of affection that he knew she would bestow even upon Jace as well. Just a small, quick, peck on the mouth to show her gratitude or simply just to tease him because it amuses her when he'd wipe his mouth and whine about how gross girls and kisses were.
He learns later with some strange joy that she never kisses Jace like that, only him.
The second time Isabelle kisses him, she's fourteen and he's sixteen, and she tastes of alcohol and something…something he couldn't quite define.
But it makes his stomach recoil and she smiles at him, one of her teasing Isabelle smiles and leaves to go to her room. Alec lies on his bed later on, his thoughts muddled as he struggles to regain control of himself, wondering when his sister has stop being innocent.
Because she tastes like somebody else and he hates it.
The third time Isabelle kisses him, it's a month after the war and she tastes of tears and bitter agony.
He doesn't stop her when the kiss turns into something more because he knew she needs something – it – this – and he obliges his little sister because he loves her and would do anything for her.
She still cries through it though and he isn't angry because she pulls him closer and whispers his name, thanking him for chasing the nightmares away.
The fourth time Isabelle kisses him, they're at a party and she tastes of champagne and strawberry lip-gloss.
He doesn't object when she takes his hand and drags him inside an empty room before she kisses him with a passion that makes him groan. He never thinks about what they did, only what made them feel, never thinking how wrong this is and how he shouldn't know how his sister gasps when he bites on her collarbone or how she moans his name when she comes.
He knew she doesn't think about it at all and he's trying to do the same.
The fifth time Isabelle kisses him – and the sixth time – and the seventh – and the eight – until he's lost count of how many times he's kissed her lips, she tastes of sin and things that he knew would be the reason they were going straight to hell when they die.
He doesn't bother to understand or comprehend how completely wrong and sick what they did is because it truly didn't matter anyway. And what matters was how she whispers that she loves him after sex, with so much emotion, it almost hurt.
And he whispers back that he loves her too before she kisses him again.
I dedicate this to the wonderful Gema227, whose Alec/Isabelle fics I adore and the reason I got so hooked on this couple.