This page of our guide explains whether you can complete Wolfenstein: Youngblood campaign on your own, i.e. without using the recommended cooperation mode with a second player.
Yes, this is possible because the co-op mode of the game is not mandatory (although it is recommended by game developers). If you don't want to or can't invite another person to play, you can start the campaign in solo mode.
When starting a new session (Host Game) you can choose to play offline - this is shown in the picture above (Who Can Join - Nobody Offline).
If you've decided to go through a solo campaign, artificial intelligence will take control of the other sister. An AI sister can take an active part in battles, help in solving puzzles, activate Pep Signal and even "resurrect" a controlled character after you have lost all your health points.
A partner controlled by artificial intelligence is not immortal. She can lose all health points, if she find herself under a massive fire from enemies. Just as in the case of a "living" partner, it is worth to help her and restore her to full strength. The death of an AI-controlled sister means the loss of one life from the common pool of lives.
If you have purchased the Deluxe Edition, you can use your Buddy Pass to invite one of your friends to play with you. The second player will be able to download the game and join the competition in cooperation mode without any additional fees. We described it in more detail in the How does a Buddy Pass work? section.