Can I use family share to send Css textures to brother's Gmod?
If you both play the game on the same computer the textures will be installed and copied over anyway. However if not you could go to this website go to the garrycontent page and scroll down to the part where it says Source Textures or something like that and add it to your
C:\Program Files (x86)\Steam\Steamapps\Common\Garry's Mod\addons
This will install the textures for you no matter what :D