MY BROTHER HAS BAD DREAMS (1972)

  AKA SCREAM BLOOODY MURDER(which leads to confusing this with that other SCREAM BLOODY MURDER from the following year with the killer with the hook-hand) this is the tale of a mentally deranged fellow(after witnessing his father kill his mum) who lusts after his sister and just might also be a homicidal maniac(spoiler alert!). This is a good example of a 70s regional(FLORIDA) horror film, but it's not exactly a very good movie. It takes a bit long before any of the real horror elements kick in and the main character is a not exactly a likeable fellow to follow. You basically get a twist on PSYCHO but if Norman Bates was retarded and really into incest. They do throw in a pinch of blood and a sprinkling of homoeroticism but the  rather wacky ending, involving some sharks!, is the most memorable thing here. 

Shown down at the drive-in with THE BLOB, which I'm guessing might have been BEWARE! THE BLOB since it also came out in '72 though the 3rd feature is the re-titled 4D-MAN so maybe it was just a couple of 50s movies thrown on.