It's very unlikely The Sims 5 will be running on Unreal Engine. It's free to use, until you make more than 1 million USD. From that point on, companies will have to pay 5% royalties over their earnings. Considering The Sims 4 just crossed 1 billion in lifetime revenue, had they used Unreal Engine, they would have had to pay 50 million USD to Epic Games. EA would never.
And also, a game like The Sims doesn't need half of the features present in Unreal Engine. By building their own engine the game will be less bloated. Which is a nice thing. You don't need gigabytes of first person shooter data in the base game.