Garrys mod family sharing help

My brother bought gmod and the stuff you need for it css/hl2/portal that stuff and when I go on my account and play gmod there is no textures on darkrp servers like no css textures or anything and its on the same computer so I can’t just install css because its already installed.

Go to the main menu of garry’s mod, in the bottom right there should be a game controller icon, click that and see if you can mount CS:S/HL2 EP1/2

No it won’t let me