Psychopath husband and drugs in a food - thriller movie

My friend looking for a movie from the 80’s or early 90’s thriller movie.

The woman has an almost adult son and daughter and a third child of several years old. Her husband died and she married her husband’s work colleague. This second husband turned out to be a psychopath and imprisoned the whole family in the house. He gave them drugs in their food, so the children didn’t eat anything because they found out about the drugs and the youngest child was given to drink his own blood, they cut themselves and the child drank it so as not to die.

In the final scene the man wakes up in a grave underground and shouts for help. Then a scene is shown that a few years after that some bush grows in the place of the grave and the family looks at it with relief.

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Could it possibly be First Born 1984? I don’t recall the ending other than serious abuse scene

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