It’s an old black and white movie, I think it was set sometime after WWI.
A wealthy British (possibly American?) woman lives in her family’s large Italian(?) villa. Her brother, presumed dead in a car accident, returns home, only it’s not her brother. She summons a detective from the local police and the brother spends the better part of the evening trying to convince them both that he is, in fact, her brother. The “brother” seems to know everything about the woman and the family that he should know, and eventually convinces the detective and he leaves.
The woman remains certain that he is not her brother, and begins to question her own sanity as mysterious events continue to unfold…
I won’t give away the rest in case this interests anyone who wants to see it (I really enjoyed this movie), but I think that should be enough to ID this movie. If not, I can go on. The title is just completely escaping me.