Margaret was born at Haus Sandberg am Alsensund near Sonderburg[1] on 24 February 1583[2] as the youngest daughter of Duke John II the Younger of Schleswig-Holstein-Sonderburg and his first wife Duchess Elisabeth of Brunswick-Grubenhagen.

[7] John's idea to give the Protestant cause good leaders for a people's army, was the reason for the Kriegsschule, founded in Siegen in 1616, probably the world's first military academy.

But despite the fact that he was so indebted by supporting the Dutch Revolt, that for some time he considered giving up his residence in Siegen and going to live with his brother William Louis, he nevertheless founded the school.

At that time Margaret wrote a letter to King Christian IV of Denmark, asking for the payment of an old debt.

[16] When John received the County of Nassau-Siegen in 1607, he decided that such a small country (it had about 9,000 inhabitants and yielded an annual revenue of about 13,000 guilders) should not be divided up again.

To avoid this, he made a will and testament, which stated that only the eldest son would rule and the other children should be compensated with money or offices.

[19] Abolition of the primogeniture would have meant a division of the small country, and therefore John the Middle opposed all proposals in that direction.

All his brothers advised John the Middle to change the primogeniture, but he firmly trusted the word, the letter and the seal of his son.

[19] Only once John the Middle was convinced that his son was under the influence of the Jesuits and that the possibility of a Catholic area within the Nassau lands was a danger to the Protestant inhabitants, did he get persuaded to make a new will.

[17] On 3 July 1621 he drew up a third will, in which he laid down something that he had always considered to be utterly nonsensical, namely to divide the small county of Nassau-Siegen, which was barely able to support one lord, into three parts.

John the Younger had the imperial decree read out, and when his brothers were not very impressed by it, he said as he stood up: "Der Kaiser wird uns scheiden!"

He had taken the precaution of obtaining a further imperial decree on 20 November 1623 against Margaret (now the Countess Dowager) and her sons, in which the Emperor strictly forbade impeding John's assumption of government, his taking possession of the land and his inauguration.

However, for a short period (1632–1635) this situation underwent a temporary change: during the Thirty Years' War, his brothers, who were fighting on the Protestant side, rebelled against John the Younger.

[21] Count Louis Henry of Nassau-Dillenburg entered the service of King Gustavus II Adolphus of Sweden on 1 December 1631, who had landed in Germany on 24 June 1630 to intervene in favour of the Protestants in the Thirty Years' War.

[27] Countess Dowager Margaret, through the mediation of Louis Henry, turned to Gustavus Adolphus and asked for help against the machinations of her stepson John the Younger.

Consequently, on 14 February 1632 the Swedish king sent an order from Frankfurt to Louis Henry to provide military support for his first cousin John Maurice.

However, the latter soon restored the old order: in 1636, he again became the sole owner of his father's property, with the exception of Hilchenbach, which he left to William, and he again governed the city of Siegen alone.

[30] On 22 January 1645, after his return from Brazil, the latter, with his brothers George Frederick and Henry and an 80-man entourage, had forcibly occupied Siegen Castle and on 15 February had received the renewed homage from the citizens, albeit this time only for two-thirds of the county.