The adjective is 'cytherean'.
'Venerean' is too much like 'venereal', 'aphrodisial' is too much like 'aphrodisiac', and 'Venusian' is etymologically unsound (like saying 'Earthian')!
Cythera (known in Greek as Kythira) is the fifth largest, and southernmost, of the seven main Ionian islands. It's off the south Pelopponesian coast; the other main Ionian islands are off Greece's south–west coast
© Haydn Thompson 2017