Make sure to download the new version of hellica (1.0) to play the Sibling DLC.

make sure to extract them both and move the files from the sibling folder inside hellica:

\HellicaNewVersion\Hellica\Content\Paks

it should be looking like this:

after that, reopen the game and let's see if you can make it alive. please use headphones and prepare to get scared.

this game is not for the faint-hearted. you have been warned.

    Hellica All DLCs document (site)