Sidonie married on 17 May 1545 Duke Eric II of Brunswick-Lüneburg (1528–1584), who was ten years her junior.

Landgrave Philip I of Hesse predicted: "All sorts of things will happen inside this marriage after the kissing month ends.

"[1] Two years into the marriage, in 1547, Duke Eric began his rule and reconverted to the Catholic faith, after the Reformation had been introduced to his Duchy in 1542.

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.

"[3] Sidonie was from 1564 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.

On 30 March 1572, Duke Eric assembled some of his advisers, nobles and deputies of the cities of Hannover and Hameln on Landestrost Castle in Neustadt.

Instead of Calenberg castle and the silver Duke Eric had withheld from her, she received, after several settlements, compensation and a pension for life.

Elector Augustus gave her the Poor Clares monastery at Weißenfels with all income and interest.