^ I think Hela being Loki's daughter is alot more certain that Sif being Heimdal's sister, because it's alot more inherent to her character and plays a role in many Thor stories.
CAPTAIN AMERICA, THOR, IRON MAN, THE INCREDIBLE HULK, BLACK WIDOW, AND HAWKEYE
Spider-Man - It's the end of the world, I'M FREAKING OUT...why aren't you freaking out?
Captain America - Because I can hear it.
Spider-Man - Hear what?
Captain America - ....thunder *cue Thor's grand entrance*