Flowers in the Attic (1987)

Other mistake: When Christopher is in the bathroom stupidly "dabbing" Cathy's back as she's taking a bath there is a glass block window behind them. The window makes no sense. It doesn't match the era or architecture of the bathroom or the home, first of all, but probably more importantly - what's behind it? Where is the light coming from? It's just this random, standing glass block window with an equally-random light source. (00:34:53 - 00:56:49)

Cathy: Christopher, what is it?
Chris: A copy of Grandfather's will. It's 2 months old. It says if it was ever proven Mom had children from her first marriage, even after he's dead, she'd be disinherited.
Cathy: Mother's known all this time that we could never be found.
Chris: She never meant for us to leave that attic.

Trivia: Victoria Tennant hated the scene where her character was to be killed off so much that she refused to film it and walked off the set. The scene then had to be done by a stunt double.

Question: It's been established that the grandfather has no knowledge of Corrine's children. When they come across a copy of his will, it states that if it was ever discovered that Corrine had children from her first marriage, she would lose everything. Since he never knew he had grandchildren why was this clause ever added?