Sandbox Siblings.
1. MC doesn't have modern technology or economics. So they have no fucking clue what a jojamart is or why Stardew doesn't like it.
2. While Stardew and MC have mechanics to fight, AC doesn't. As a result: Scary Dog Privileges.
3. AC often has his phone, so plenty of selfies are taken. Stardew knows what a phone and camera are, MC doesn't.
4. Stardew can't jump gaps and MC can only jump so far. So the bridge builders are introduced to the concept of a vaulting pole and heart attacks.
5. AC loves to dress up and make outfits, and now he has two siblings who would do nearly anything for him and are not against trying on a few clothes.
6. Tending to bases, farms, and islands are hard work. So the siblings hunker down for a hard earned nap.