The wedding ceremony was held in Hann. Münden without the usual pomp and circumstance. Initially, they liked each other. Eric had been engaged to Agnes of Hesse. When the marriage was negotiated at the court in Kassel, however, he had met Sidonie. He liked her, and broke off the engagement with Agnes, in order to marry Sidonie.
Landgrave Philip I of Hesse predicted: "All sorts of things will happen inside this marriage after the kissing month ends. Two years into the marriage, in , Duke Eric began his rule and reconverted to the Catholic faith, after the Reformation had been introduced to his Duchy in Despite her husband's pleas, Sidonie held on to her Lutheran faith.
They had financial problems and the marriage remained childless marriage, and soon their relationship took a very unfortunate course. The clashes culminated in her suspicion that her husband wanted to poison her. A Genoese merchant had contacted Sidonie's brother Augustus in and informed him that Duke Eric I had ordered poison from him on the grounds that "Eric was a Christian and his wife would be Lutheran, it was better that one woman part was destroyed, than 20, people.
Sidonie was refused access to the castle, which was also based on the grounds that she had threatened "if she comes into my house, I'll cut the whore's nose off and poke out an eye. Sidonie was from onwards virtually under house arrest and she protested vigorously to her brother and to the Emperor, who sent councils who tried unsuccessfully to compromise with Duke Eric.
In Eric fell very ill and suspected he was poisoned. Four women suspected of witchcraft were burned as witches in Neustadt am Rübenberge. In mediation by the Emperor, the Elector of Saxony and Duke Julius of Brunswick-Wolfenbüttel resulted in a settlement of the disputes between Sidonie and her husband, in which Sodonie would receive Calenberg Castle.
