Not able to add members to existing dynasties

I tried to add Arnessa Thebe to the Thebe dynasty but there are no sims showing up for Cordelia to add. I even made her and Arnessa best friends and lovers (ugh) but there is no way to add her in sadly. If that is how the game is intended to work it's really sad that important familys and family members from other packs are not included in the dynasty mechanic.