Hi final,
I understand what you're saying. Are you using Bitlocker to password protect your entire hard drive?
And so you understand, you have NOT physically secured the hardware if it's in a location like a bedroom or closet or something if he can just walk up to it and plug in a usb flash drive!!! If you want it physically secured, that means like putting a lock on your bedroom door where he can't physically touch the computer.
I've never seen anyone try to unlock a hard drive protected by bit locker with a usb drive; it might be possible. However, depending on the type of computer or laptop you have, there are other methods. If you have a more modern PC or laptop, they have something called UEFI BIOS *you can Google it*, which prevents ANY external devices including usb drives from accessing the hard drive or Windows. In UEFI mode enabled, and the boot in SECURE mode, no external or internal devices (cd/dvd or usb flash drive) can be booted to upon computer bootup. That means NO access to the hard drive or Windows from those or any other devices inside the computer or plugged into it (including floppy drives).
If you don't know whether your computer has UEFI BIOS or not, you can download the computer user manual for free from the Manufacturer's website (e.g.: Dell.com), and it will tell you this. Or ask a Tekkie friend or take your computer into your local computer repair shop and then can tell you in about 5 min. Of course this will cost you some $$ in some shops.
If your computer or laptop is older (more than 3 yrs. old); and does not have UEFI BIOS, you can still perform some lockouts. Go into the BIOS, and disable all settings with the term USB in them; especially, LEGACY USB. You'll have to experiment with your specific computer, because disabling all of these settings may disable your keyboard and mouse and in some cases your touchpad (**Ouch!**); so make sure to write down the settings that are there before you change them or take a photo with your camera-phone. In many older computers, disabling USB can be done for individual USB ports; say usb1, usb2, etc. You can disable all of them one at a time. In other computers, disabling USB can shut off everything like I said. Usually, disabling USB ports will keep a usb drive from booting, but sometimes you'll also have to disable or remove them from your BOOT ORDER setting. Most computers won't let you remove them, but rather disable that device from booting and bypassing the standard windows boot from your Windows boot hard drive (the C: drive).
Given this information, all of these tricks won't work if your brother has physical access to the PC or laptop, as if he has that he can take it apart, take it to a tekkie friend, or even a computer store to bypass these settings. Physical access restriction is the best way to keep him off your computer! Put your computer or laptop in a locked room to which you only have the key!
Lastly, one other trick is to put a Password on the BIOS Administrator account. In most computers, this will automatically force any bootable devices (such as a usb flashdrive), from being accessed withouth the BIOS Admin password. However, take great care to write this password down as clearing or resetting this password if you forget it will cost you $$ to get it fixed.
Hope that helps.
<<<BIGBEARJEDI>>>