When the brothers were both little, Odin cursed Loki to an eternal sleep (like Sleeping Beauty). He left Loki in a glass casket in the vaults and told everyone that Loki died of a fever or some rot.
The fic I'm looking for is actually a two parter. Very possibly by two different people.
Anyway, the first part was a short ficlet. It explained what happened and had Thor basically coming down to the vault to visit his sleeping brother.
The second part was longer and shown how Thor grew up knowing that his little brother was in the vaults like a discarded thing. That made him grow up differently, he was less anger-prone and thought more. And he plotted, all his life, how to free Loki of the curse. He roped in the Warriors Four, he met Helblindi (found out Loki was actually the youngest of Laufey's kids), married Sif, had a son, and eventually got crowned King. First thing he did was free Loki. Because of the curse, Loki hadn't aged at all, so now Sif and Thor are raising both Magni and kid!Loki as their precious sons.
I'd love to read them, so please help.
EDIT: Nevermind, I remembered ConeyCat wrote the second part: it's called True Love's First Kiss http://archiveofourown.org/works/340755
There a link to the first part as well.