Der geheimnisvolle Schwiegersohn (2007)

The daughter of a famous writer believes her mother is responsible for her father's death. Since then the relationship between them is cooled increasingly, to the point that she did not invite her mother when she marries. The husband, far from promoting good relations between them, takes them away even more, which sees the mother and begins to suspect him when he receives a note threatening to kill his daughter.

Unfortunately the movie Der geheimnisvolle Schwiegersohn is not yet available on HBO Max.