Shud i tell him?

iv been with my bf for 4 yrs and we have recently been really gd friends with his cousin and his gf (the have a 1 yr old baby 2geva). a few months ago we all went out and had a few drinks and i got a tx msg frm the cousin sayin he wished i was his gf. i really do have feeling for him but he dusnt no that. i dnt fink i cud have a relationship wiv him because im gd friends wiv his gf and they have a baby. i love my bf 2 bits and wudnt want 2 hurt him. i lke a flirt tho. we're goin on hol wiv them in 2 months and was finkin about tellin the cousin that i lke him but i crnt do nefing about it. wat do u fink?? plz help