The Old Castle's Other Secret or A Letter from Home

"The Old Castle's Other Secret", alternately titled "A Letter From Home", is Don Rosa's sequel to "The Crown of the Crusader Kings". The title is a reference to "The Old Castle's Secret", which was the second story to feature Scrooge McDuck and was the first in which he was a starring character in an adventure with his nephews.

Plot
Scrooge McDuck and his family return to Castle McDuck to find the treasure of the Knights Templar; the caretaker is revealed to be Scrooge's sister (and Donald's and Huey, Dewey, and Louie's aunt and great-aunt respectively) Matilda McDuck, who has not spoken to her brother for nearly 25 years.

Matilda, upset at the situation, reveals that she already knows about the treasure; it was their father Fergus's wish not letting Scrooge know about it; both Scrooge and Matilda think it's because their father didn't approve of him and his choices. Scrooge, despite being hurt and depressed, nevertheless keeps searching for the treasure. However he is being shadowed by Mr. Molay and a reluctant Maurice Mattressface, who have stolen the Crown of the Crusader Kings, which is the final clue to find the treasure; eventually Mattressface rebels against Molay, who reveals himself as a member of the villainous Priory of Sion. After reaching the treasure vault, a confrontation between Molay and the others ends in his defeat (with Donald knocking him out by breaking the Holy Grail on his head), while Scrooge puts himself in harm's way to protect Matilda.

The treasure is found (and given by Scrooge into Mattressface's care to be devolved in worthy causes), as well as a letter left by Fergus telling the real reason he didn't tell Scrooge about this treasure: he felt that Scrooge would have felt better making his own fortune instead of inheriting one, while also stating his (and his wife's) pride in him and his accomplishments. While alone with Matilda, Scrooge admits that the reason he never tried to make peace with her was because he was ashamed of his actions from the last time they talked, and that his ideals hadn't stood up to his travels from building his financial empire (so his money, instead of being trophies and rewards for his work and exploits, became only profit to him); he also confesses that he envies Donald for having a family, but the McDuck spirit of adventure he sees in his younger relatives has inspired him to return to a life of adventure ever since he first met them. With this confession on behalf of Scrooge that he does care about his family, he and Matilda make up at last.