In Stockholm 1 October 1559, she was present at the wedding between her stepdaughter Princess Catherine and Edzard II, Count of East Frisia. On their departure from Sweden, the newly weds was accompanied by the sister of the bride and the brother of the groom, Princess Cecilia of Sweden and Johan II of East Frisia. On their stay in Vadstena, Cecilia and Johan were discovered by Crown Prince Eric to have a sexual affair. The great scandal were to be known as "Vadstenabullret" (The Vadstena Thunder). Catherine and Edzard were placed in house arrest, Johan were imprisoned, while Eric and Cecilia where both called back to Stockholm. The King is said to have cried when he related the story to Queen Catherine. During the whole affair, Catherine Stenbock were on numerous occasions asked to act as mediator between the King and his children. She accompanied Cecilia to her confrontation with her father after her return to the capital, and she acted as channel through the King for Princess Catherine, who tried to negotiate Johan's release and permission for her and her spouse to leave for East Frisia. In May 1560, Prince Eric asked her for a permission from his father to travel to England to propose to Elizabeth I of England. King Gustav fell sick and died during the scandal. According to the chronicles, Catherine was present by his bedside during his entire sickness until death, had a bed brought in by his side where she slept, and eventually fell sick herself. She continued to act as mediator between the King and his children, and upon his complaints that they were not there, answer that they would have been, and wished to, had they not been so afraid for his anger. King Gustav died on 29 September 1560 and was succeeded by his son in his first marriage, Eric XIV. In his will, Catherine was promised custody of the Princesses, who were to live at the court of Eric or her until they married, an allowance suitable for her position, and that his sons should always see to it that she had her needs taken care of. He did not specify which estates should be given to her in his will, but prior to his death, he had his two eldest sons Eric and John promise to grant her preferred choice, Strömsholm. In March, Catherine had Eric XIV issue a written statement of her right to Strömsholm, which remained her seat for the rest of her life, as well as several other lands and parishes in Sudermannia, as well as her own residence in the capital.