So at first I was a little ??? about Hela being Thor’s sister in Ragnarok (squeezing her into the role Angela so recently acquired as their long-lost-big-sister in comics), but the more I think about it, the more I like what it does for Thor and Loki’s arc.
Thor now has two siblings who became his antagonists because of his father keeping secrets – hiding Loki’s heritage, and then hiding Hela’s existence. Which re-enforces how damaging that habit of lying and secrecy is to Asgard’s growth, as represented by Thor. Having those secrets come out and be faced is necessary for Thor’s development and maturity – confronting the sins of his father.
And for Loki – when Loki tries and fails to be a hero, he becomes a villain. He always measures himself against Thor, and then casts himself as Thor’s opposite. But with Hela showing up, suddenly the role of bad guy has been usurped by another sibling; he’s no longer the baddest Asgardian, or even the baddest of Odin’s kids. His sins are now in a whole new context, where his misdeeds are frankly small potatoes. He’s not only been outstripped as a hero by his sibling – he’s been outstripped as a villain. And that forces him to find some other measure of identity; not wholly good, not wholly evil, but something in between – something new.
Also, it re-enforces their brotherhood in an interesting way. Hela is Asgardian. Hela is Thor’s blood sibling (or at least half-sibling). And Hela is still awful. In that light, Loki can no longer ascribe his wickedness to his heritage – he isn’t evil because of some innate genetic factor, or because he isn’t Asgardian, since Hela is clearly capable of that evil despite being raised on Asgard and having Odin’s genes. And while Hela and Thor share blood, they have no kinship to speak of. Thor and Loki do, despite the lack of blood relation. They snipe and bicker like brothers throughout, and there are callbacks to their childhood and past together (the snake story, ‘get help’).
Hela’s appearance as Thor’s sister lends new context to both Thor and Loki’s relationship with each other and their family, and I think it gives us, as fandom, a lot of fresh material to play with as far as our boys’ character growth moving forward.