Hela's parents in Norse mythology are Loki and a Jotunn sorceress called Angrboda (or Angerboda). They were her parents originally in the comics too until Jason Aaron retconned Hela's back story and made her a 'creation' of Loki rather than his biological child. In Aaron's run kid Loki created his 'perfect friend' which he called Leah and wrote her into existence, and then a load of **** happened and Loki ended up asking Hela to send Leah far back in time to protect her from his own evil self and it turned out that Leah grew up to become Hela, so Hela actually
sent her child self back in time, creating a neverending time loop... Ya gotta love comics.

I guess it explains why Hela often looks older than Loki and is more powerful than him though.