@Vortico Great suggestion
If you’re going to have an external there all the time- you might as well boot off your internal drive- it will be faster- and store your data on the external.
It’s actually a pretty smart setup to boot off an internal drive not cluttered with all your data. Make sure your external is a fast drive though with a fast connection since you will be accessing it constantly- and probably one with a good well ventilated enclosure that is quiet. In this situation it’s probably worth it to pay more for a really nice external drive- and store it well- I’ve had externals hooked up semi-permanently and accidentally knocked them off the desk. (And destroyed them!)
The only other downside is since the computer has multiple users will be teaching your brothers as much about the setup as you know. Like make sure they know not to unmount the external etc- or that if they want to go root around in their home directory to pick that drive not the internal.