United Duchies of Jülich-Cleves-Berg
Her brother John William inherited the United Duchies of Jülich-Cleves-Berg in 1592. After he had developed a mental illness, a power struggle broke out at court between Sibylle and her sister-in-law Jakobea of Baden. Sibylle won, and imprisoned Jakobea. Sibylle may also have been partially responsible for Jakobea's violent death in 1597.