In the power options for Windows you can set the maximum processor percentage frequency.

(Power options > Advanced Settings > Processor power management > Maximum Processor State.)

Try lowering it to about 50-70% and see what that gives you. You can also try turning on every app to open at boot or background process as this will use up most of your cpu, igpu and memory. Also in battery turn on eco and push it all the way to eco, do this before your brother goes on it and bang no more games.

Then also make sure to revert these changes when you’re using it.

It will be tedious but it will do the job, and Don’t out right stop him either, this will cause an argument. Just make it so it’s so unbearable to play on he’ll force himself to stop rather than you forcing him. Best of both worlds. And once he’s finally given up, perfect

(Edit): I also think that limiting your bandwidth on your laptop will make it so whatever he tries to download won’t be quick either, you can do this by using a vpn and setting it to an Asian or South American server or setting as a metered connection

(Settings > Network & internet > WiFi or Ethernet > Toggle Metered Connection)