My sims 4 app won’t Launch from the EA app and says I don’t own the game.

@ElliottandSophie  Try clearing the EA App's cache and restarting your computers, one at a time.

https://help.ea.com/en-us/help/faq/clear-cache-to-fix-problems-with-your-games/

However, this might be a limitation of the EA App itself and its protections against account sharing, which is technically against EA's terms of service even within a single family.