Can you identify this horror/thriller movie?

This is an old B&W movie that used to appear on TV late at night in the 60's. Probably considered a grade B picture. I'm unsure of many of the facts. A young man shows up for dinner. There's a sister, a brother, the stranger, and perhaps the old mother. The stranger reveals that he's not a stranger, but the adult version of their brother who ran away from home when he was young, say 10. I'll call him Tommy. The sister believes him, the brother doesn't. Tommy keeps revealing facts that unnerves the brother. The tension rises in the house. The brother says menacingly to Tommy, 'I don't know who you are, but you're not Tommy'. It turns out the sister is in on it, feeding Tommy the information, and she's using to unsettle the brother. I'm not sure of the motivation here, perhaps inheritance? Towards the end of the movie, the brother is deranged, and tearing out the wall, perhaps the wall behind an organ?, and the house is burning? or otherwise disintegrating, and the camera focuses on the hole in the wall, which reveals the mummified remains of Tommy. Tommy hadn't run away, his sibling had killed him and hid in the wall years ago. I thought the movie starred Oliver Reed, but I can't identify it.