Looking for a 60s' or 70s' horror movie!?

I watched this during a very dark period in my life, so i only remember key parts. Brother and Sister Inscest relationship and they end up living on an island, loads of bad scenes but the death scene at the end rules. THe brother is thrown into a lime pit and is dying as his sister gets it on with some caveman looking doode. Someone help me find this movie, please!