Told you it would be soon!
The two young princes trained often, sometimes with their father, sometimes with teachers, but mostly just together. They soon began to draw attention from other Asgardian children, and many of them would sit in the tall trees surrounding the clearing to watch. Thor enjoyed this audience immensely, and would try to get the other boys to come down and practice with them. Loki on the other hand hated it, feeling that most of the children watching them were only there to witness Thor's brilliant skill in just about every fighting technique that existed. Worst of all though, was that most of their audience consisted of girls, who would giggle and whisper in horribly off-putting ways.
It would be several years before Thor could convince anyone to train with them- with skills far beyond their age the other Asgardian boys were secretly scared of the brothers. But one day, when Thor was shouting something up into the trees about needing a new adversary, a small figure jumped down from a high branch, and approached them. Both boys were somewhat unsettled by this development, for it was not one of their friends, but a girl, with blonde hair that fell past her waist and shone in the sun. She smiled at their confusion, and said to Thor "Do not look so put out! It is not only boys that can fight, you know. But I do not wish to train with you." She turned to Loki, and, gesturing to the throwing knife that was still in his hand, asked "How do you use those? Will you show me?"
In the months that followed, Sif trained with them often, and Loki did indeed teach her to use his knives. After that they practiced evasion, first by throwing a set of blunted knives at each other, and then moving on to the real thing. They played and explored together in the city as well, and Loki, who had at first been desperately shy around her, began to talk, to open up to her. He confided in her about his jealousy of the adoration Thor received everywhere he went, about Odin's perpetual disdain for his endeavours, even confided in her his developing fascination with magic. They became close friends, but after a while whatever it was that had sparked her interest in fighting gave way to the attraction of joining the other girls during their training sessions, watching and giggling as the boys fought.
Now though, Loki felt at ease, and even enjoyed being under their watchful eyes, as he knew at least one of them wouldn't be mocking him. At least he did, until the day that he and Odin had a particularly nasty argument about a book Loki had been caught reading. The book was about dark magic, and no amount of pleading innocent curiosity could calm Odin's fury. Loki had run away from his father, and looked high and low for Sif, every hiding place and favourite spot they had. Failing at that, he searched instead for Thor, who wouldn't understand, but would do his best to make his brother feel better. Again though, Thor proved impossible to find. Loki had one last place to look, but was beginning to weight up the likeliness of his mother taking his side were he to return home instead.
Climbing down a rocky outcrop next to the bifrost, Loki wondered, if his brother really was in their favourite and most dangerous hiding place, what could have possibly brought him there. Neither could quite remember how they found it, this cave under the rainbow bridge. To reach it one had to climb down a steep, rocky incline, onto a tiny ledge, swing underneath it, grab another rock for support, and without falling into the rough waters below, push upwards into the cave.
He proceeded cautiously down the damp and slippery rocks, and was about to push himself up, but stopped dead when he caught site of the inside of the cave. Thor was indeed there, and so was Sif. Sitting next to one another, Loki saw that they were holding hands. A burning, sick feeling spread from his stomach through his whole body. As he watched them slowly, shyly move their heads together, he pushed backwards, contorting himself back up onto the ledge to avoid witnessing their kiss, before running back to the palace and sneaking in through a secret entrance to avoid his father.
Loki stood still in the dark silence, a shaft of moonlight reflecting off the knife in his hand. It was lucky, he thought, that his father hadn't caught him with that book until after he gleaned several methods of concealment from it. Sneaking into Sif's bedchamber had, as a result, been easy.
Although now, as he looked at Sif's beautiful hair lying in locks all over her bed, Loki noted that the sick feeling from earlier had not gone away. Revenge was attractive, that was certain. But he wasn't sure he felt any better, and was certain he'd be found out. Who else could it have been? Even his own father would suspect him immediately. He resolved to fix his impulsive decision, to find a way. Perhaps if he did it quickly enough Sif would forgive him.
"You're not concentrating!" Thor laughed, as mjolnir collided solidly with Loki's chest, sending him flying backwards into a tree. Coughing, Loki laughed too, and replied between winded breaths;
"Apologies, brother- I thought it might be good for your pride to allow you to hit me for once." Smiling wickedly, he dodged another attack, only to find that Thor had anticipated perfectly the position Loki would move to, and was already there, a blow ready to collide. "Much better!" he gasped, getting to his feet.
Loki's thoughts had indeed been elsewhere. It was impossible not to think of Sif, of their friendship, her relationship with Thor, and Loki's horrible revenge, when she was watching them, high up in a tree, just as she had done many years ago. Of course, she didn't forgive him for his actions, and when he failed to undo the damage their friendship looked to be ruined forever. It would have been, were it not for Thor's enduring loyalty to his brother- Loki knew not what Thor had said to her as they spent their adolescence whispering and sneaking off together, but Sif eventually began to speak to Loki again. Of course, that was long before Loki learned of his true roots. He had no desire to know what she thought of him now.
Sif shifted along the branch she was perched on- this was much more uncomfortable than she remembered. As she watched Thor and Loki train she was also thinking of their past together. Loki didn't, and never would know the real reason she tried to forgive him, such a long time ago. Sif's mother was a powerful woman, strong, fearless, and although lack of opportunity resulted in her never actually becoming a warrior, she would have had the perfect constitution for it. She regarded her vain, self-absorbed and beautiful young daughter with a great deal of disdain, and eventually tiring of Sif grieving over her spoilt hair, her mother sent her away for a time to learn to become a warrior. It was this that transformed Sif into the formidable opponent that she was, and were it not for Loki disfiguring her in an act of selfish rage, it would never have happened.