Complex sentence: Another detail which can easily escape is that 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.
Split into simpler sentences: Another detail which can easily escape is when Madeline is showing Philip the crypt, her two great grandparents, two grandparents and two parents. In retrospect this shows that inbreeding has resulted in the Ushers following a direct line with no deviation.