[1] Sibylle Ursula was born in Hitzacker, the third child of Duke Augustus (1579–1666) from his second marriage with the Ascanian princess Dorothea of Anhalt-Zerbst (1607–1634).

Sibylle Ursula became the stepdaughter of Elisabeth Sophie of Mecklenburg (1613–1676),[2] the third wife of Duke Augustus who had distinguished herself as a poet and composer.

Like her siblings, the Brunswick dukes Rudolph Augustus and Anthony Ulrich, she received a comprehensive education at the Wolfenbüttel court by scholars like Justus Georg Schottel and Sigmund von Birken.

[4] Translations of hers include two novels (Cassandre and Cléopâtre) by La Calprenède, parts of Scudéry's Clélie, and Introductio ad sapientiam, one of the Latin writings of Juan Luis Vives, a Spanish humanist.

Her husband, the only surviving son of Duke Philip of Schleswig-Holstein-Glücksburg, had taken over the rule at Glücksburg Castle the year before and was able to restore public finances with the help of his Wolfenbüttel relatives.