E.R.
Boy, that's a tough one. It sounds like a game to me. Can a well fed 2.5 yo really be that hungry in the middle of the night? My son was small and I would let him eat often; it's hard to say no when they want to eat! But, barring any health issues that might require a wee hour meal, I would stop playing the game. Tell him that people don't eat in the middle of the night and he will have breakfast in the morning when everyone else eats breakfast. Maybe give him a filling snack before brushing teeth and bed. I'm sure he will protest loudly for a few days and wake his sister. Stay firm and I hope his habit goes away quickly!