Original: Seymour's ambitions led to his being arrested and beheaded; after which, Sudeley Castle became the property of Catherine's brother, William Parr, who was the Marquess of Northampton.
Simplified: Seymour's ambitions led to him being arrested and beheaded, after which Sudeley Castle became the property of Catherine's brother William Parr, who was the Marquess of Nothampton.