the situation in hungary was not stable. george martinuzzi was assassinated in december 1551, while the ottomans captured cenad and unsuccessfully besieged eger. isabella received invitations both from sultan suleiman, who sent an envoy to poland, and local nobles to return to hungary, but she delayed. her brother king sigismund ii augustus, afraid of a habsburg–russian alliance in the series of the muscovite–lithuanian wars, married catherine of austria, daughter of ferdinand. isabella and her mother bona sforza demanded that ferdinand fulfilled his obligations according to the treaty of weissenburg, but he did not have financial resources to comply and asked for concessions. isabella decided that since he did not hold up his end of the bargain, she was not bound by the treaty and resolved to return to hungary.