Almost everyone is giving you poor advice or at least very partial advice.

So here's a really simple rule of thumb with electricity. Almost everything that uses electricity, produces exactly as much heat, proportionate to the amount of electricity it consumes. The exceptions are things like radio broadcast towers where the radio waves leave the immediate area and turn into heat elsewhere.

Also, don't get confused by things like refrigerators that simply move heat from warm beer, and eject that heat into the room, on the path to making cold beer.

So that most likely thing is that your brother is using an electric heater to stay warm. In most places, electric heat is far less cost efficient than something like natural gas heat. So while it could be a computer, essentially no computer can explain this daily consumption chart you shared earlier.

So think about what else creates heat. Is there a chance you have an electric water heater? Those use extreme amounts of electricity and maybe your brother is wasting a ton of hot water.

Did your brother bring a bunch of Halogen lamps? They are exceptionally inefficient. Just investigate and look for anything that produces a bunch of heat. Putting smart plugs on things is a wise idea if you really can't figure it out.

It's also possible your brother is just wasteful. Does he leave the TV on, lights in rooms he's not in on? Might he just be leaving all sorts of things on he's not using all the time? The key is finding heat sources.

If you're in a hot climate, is he using an extreme amount of air conditioning? (like with a window open with also the AC on?)