Reset Windows 10 from the settings app. That should make it totally new, deleting all existing programs.
If that doesn't work (you seem to write that) download a totally new version from https://www.microsoft.com/software-download/windows10 and install that.

Then make the account of your brother a standard account in stead of administrator, and he can't install anything anymore. Make 2 administrator accounts for yourself, one for normal use, and one spare in case the normal one gets corrupted.