A Delicate Balance (1973)

Agnes and Tobias are an upper-class Connecticut married couple whose relationship has been uneasy for many years, since at least the time their son died; but they've managed to find a certain comfortable pattern of uneasiness. Agnes's sister Claire lives with them and insists that her perpetual drinking is not alcoholism but willfulness. Their daughter Julia, poised to have her fourth divorce, has come back home. Unexpectedly, her room has been taken over by Harry and Edna, Tobias and Agnes' best friends. Seized by a nameless terror that propelled them out of their own home, Harry and Edna have decided to stay.