There is a British movie, from the 1960s, (black comedy) where a killer shows up and the old man (dad) knows he killed someone. Somehow he is allowed to live there and the "old" daughter likes him but also the "old" gay son likes him to. So both the brother and sister are having an affair with the killer. The killer finally kills the old dad and at the end, the brother and sister agree to share the young killer.
WHAT'S THE NAME OF IT??
Its either late 60s or early 70s. The old gay guy drives a late 50's Buick convertible which is unusual. Its also in color!

Entertaining Mr. Sloane (1970)
Starring Beryl Reid, Harry Andrews, Peter McEnery
IMDb synopsis:
Sloane, a handsome, sexy and completely amoral young man, joins Kath's household as a lodger and proceeds to manipulate her and her brother, Ed. He is recognized by Kemp (Dadda) as the murderer of Kemp's former employer, whereupon Sloane murders Kemp. Sloane's "just desserts" are not what one would expect.

Entertaining Mr. Sloane (1970) - IMDb (http://www.imdb.com/title/tt0065700/)