The characters of the "Thor" comics series belong to Marvel Comics. I use them here only for my own amusement, and not for any financial gain.
There used to be a time when he didn't know they had separate names. It wasn't Thor and Loki, it was ThoranLoki, one word, like that. That was when his brother was a strong tree, and he was a sapling, protected in its shade. A tall golden tree, and a little green sapling.
Then it started being ThoranLokianSif, and wherever they went, it would be two blond heads bobbing ahead, with the one darker head, always a couple steps behind. Loki can fight trolls and steal horses to race before dawn, but Sif likes to do it. After a while he starts to make excuses: He has his studies, there's an important incantation he has to memorize. Thor and Sif get in the habit of not even asking before they go out without him, and Loki, he gets in the habit of watching them go.
Green eyes that held only mischief, now show calculation. The smile he used to give his brother naturally, now comes with an artifice that Loki is just learning. As for the smile he gives Sif? That doesn't come at all. What can he do to the golden girl, he wonders? What will bring her down to his level, and make her flawed like he is?
The idea, when it comes, seems inevitable: Footsteps silent as only Loki knows how to make them, a knife enchanted to perfect sharpness. Sif sleeps soundly as she does every night. And so does Loki, after he's finished the deed.
Next morning, he wakes to hear the girl sobbing. He sits up, rubs his eyes with the same childish gesture as always. Sif's with Thor, at his bedside. "How can this be?" His brother shows him a handful of golden curls. "Sif says you did this to her, brother. Tell her," he says, "tell her it's not true." Loki smiles without saying anything, and enjoys the sight of her shorn head.
His brother forgives him, of course. It's just mischief, he says. Loki's always been playful. But curls or no curls, he still spends all his time adventuring with Sif, and he comes to his brother's chambers more and more rarely. And Loki starts to wonder if he did the right thing: Was there something else he could have done to Sif, he thinks, that would have taken her out of his place in his Thor's life, that she's stolen, forever?