Saw this on Bravo when Bravo still showed art movies. It had subtitles. R
Sister is electrocuted while in a bath (milk, I think). Her brother, possibly her twin, puts her body in a bass fiddle case and drives around the countryside.
I think he might have started physically transforming into his sister, but the movie is starting to fade. The bass fiddle case on top of the car is still vivid, though.