When Madeline is showing Philip the crypt, she reveals that she had only two great grandparents, two grandparents, and two parents, showing that inbreeding has resulted in the Ushers following a direct line with no deviation.
She had only two great grandparents. This is what she reveals. She had two grandparents. This is what she reveals. She had two parents. This is what she reveals. Inbreeding has resulted in the Ushers following a direct line with no deviation. This is what she reveals. She was showing this. Madeline is showing Philip the crypt. Another detail is this. This detail can easily escape.