If you were to line up the Odinson children and play a game of 'one of these is not like the other', Thor would be the one that looks out of place. Hela and Loki look like siblings, with dark hair, pale skin and green eyes. The striking similarity in appearance between Tom Hiddleston's God of Mischief and Cate Blanchett's Goddess of Death is interesting because Hela is Odin's first-born child and Loki is adopted. Sure, there are probably multiple people who look like that within the nine realms, but because this is a comic book universe, there's a theory to explain these similarities and it all has to do with Loki's magic and shapeshifting.
The theory posted on Reddit goes like this: In Thor: Rangarok, Loki gets into a fight with Tessa Thompson's Valkyrie. Upon realizing that she is one of the Valkyrie, he mentions the Massacre of the Valkyrie at the hands of Hela and what a painful memory that must be for her. He then puts his hand on her head and, using his magic, forces her to relive that slaughter (in a positively gorgeous scene). This memory magic opens up an interesting possibility. When Odin first found Loki, abandoned as a baby on Jotunheim, Loki changes from his Frost Giant blue to Asgardian pink. The theory is that baby Loki was able to see Odin's memories, including his imprisoned daughter Hela, and he manifested his appearance to look like her.
This is an interesting theory, and I like the idea that Loki was able to see Hela in Odin's memories and changed his appearance in an effort to tug on his heartstrings and sentimentality for a child lost. Of course this would have to have been some sort of subconscious defensive, instinctual magic because Loki was just a baby at the time. In Thor, it seemed like it was Odin's magic that turned Loki so that he could raise him as an Asgardian, especially since Loki didn't know he was a Frost Giant until he was an adult. This actually opens up another possibility that it wasn't Loki who made himself look like Hela, but Odin, to remind him of his daughter.
Another reason for the similarity in appearance between Loki and Hela is a much more practical one. Although origin stories in comics can fluctuate and change,Hela is the daughter of Loki and the giantess Angrboda in both the comics and original mythical tales. So in that instance, Hela looks like Loki and not the other way around. Ultimately it seems like the similar appearances are just a coincidence, but this theory fits well enough and doesn't seem to cause major conflicts with anything else in the MCU. So if you like it, you can go ahead and call it canon.
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