They could be husband and wife, but I think they are brother and sister.
A con artist male/female team want to steal the inheritance of a mentally challenged man by pretending to be him.
Con-artist brother has blond hair; mentally-challenged man has dark hair (this is important). May be the same actor. Con-artist brother dyes his hair black and pretends to be mentally challenged.
Pivotal scene #1: Brother lays back in the bath; camera is directly above bathtub. We, the audience, can see the hair dye dissipate in the water; he can’t. An emergency gets him out of the bath, and — unaware that the dye has left the back of his head — he is almost gets caught.
Pivotal scene #2: He falls through the glass ceiling of a greenhouse and becomes mentally challenged! Family accepts him as the brother.
That scene horrified me and haunts me, and I think it was supposed to be a comedy.