In 1990 I watched a black and white horror film I would say it was set between1960-70’s.
The film was about a couple, the wife was pregnant and their niece came to stay with them because her parents died.
The neice was acting strange and the uncle became a little obsessed with her and discovered unusual bite marks on her. He follows her one night and sees like a ritual with fire going on and then he blanks out and wakes up with the same marks as the neice on his shoulder.
The aunty is getting worried about the both of them, the ending scene is the pregnant aunty on the alter being killed by her hubby and neice and a load of other people.
This is has been bugging me for a very long time please help x