Can i play singelplayer games offline when my cusin is playing on steam family sharing ?

Try and tell us, i guess yes.

If I remember, the announcement was that if someone else was online at the same time as you and using your shared library, you can't play anything of yours.

not sure.. go ahead and try it
let us know how it works :)

offline? how would it know?

wait, are you the one sharing or him?

It works here, my bro plays Starbound offline from the game files itself while I play my games.

Steam says that you can play offline ONLY GAMES WITH LASTEST UPDATE - so ye, you can play single on even 10 accounts if all got shared library*, but only one of them can be online ;P

*(correction)

Family sharing doesnt work if you're offline, iirc.

I think they removed the offline play glitch in the last update. Seems it wasn't intended like that.

Well, as long as the "lender" is offline, you can play his games. Only question that might arise is whether or not said lender is willing to go into offline mode...