Conversation so far:
What was her inheritance?
became the Duchess of Aquitaine.
What else can you tell me about her inheritance?
As these were the days when kidnapping an heiress was seen as a viable option for obtaining a title.
So there was a fear she would be kidnapped?
The duke also insisted to his companions that his death be kept a secret.
Why is that?
Rather than act as guardian to the duchess and duchy, he decided to marry the duchess to his 17-year-old heir.

Question: Why did he do that?
Self-contained rewrite: Why did the duke marry the duchess to his 17-year-old heir?