Prevent uninstalling game via Steam

I need a way to prevent removing a game via Steam. I shared my computer with my brothers, and while I am already able to disable the "Add or Remove Programs" option in Control Panel via Group Policy, this did not prevent another user from right-clicking the installed game and removing it without administrator consent.

Is Steam able to control this, or is there another way via Windows to block this kind of attempt?

You could attempt to make your own Steam skin, though I don't know if the context menu can be changed that way -- it may only be possible to edit the style of the menu and not the options it contains.

Beyond that you can (best to worst):

• Install your games to an external drive and set it up as a Steam Library Folder, then when you use the machine make plug that drive in (and make sure it is mounted properly) before you start Steam.
• Use a different OS-level user account. With yours separate from theirs unless they have admin access they would not be able to even access your data files to manually delete installed games, let alone use the Steam client to have all the hard work done for them.
• This one could cause problems for the Steam client/the game itself, but you could setup a script to automate switching back and forth and use that before/after using the PC, and they could still circumvent this using admin access: Change the Steam/steamapps/ folder to read-only (when they're going to have the machine).

Make a "guest" account in your PC and create a new Steam Account for your brother and enable Family sharing with him. That way he wont be able to access your Steam account nor your PC account so he can't uninstall anuything (since he's in a different user in your PC), but he will be able to play all your games. In fact "steam family sharing" was created with this purpose in mind: http://store.steampowered.com/promotion/familysharing

Steam has no built in way to prevent the uninstallation of games. Depending on the way the Steam uninstall script works, there might also be no way to do it with Windows administrator controls.