If she was Mieszko's sister, it is likely that they did not share the same mother, assuming that the estimated birth dates of Mieszko and Adelajda are both accurate.
It is probable that her second marriage was arranged in accordance with the Magyar tradition that the oldest male relative should marry the widow of a deceased relative (originally polygamously) and take care of his children. In 965, Prince Mieszko formed an alliance with Boleslav I Duke of Bohemia, confirmed by his marriage to the latter's daughter, the new queen influencing her husband's baptism at Easter 966 and being instrumental in the start of Poland's conversion to Christianity.
However, as noted below, if it is correct that Adelajda, wife of Geza Prince of Hungary, was the sister of Prince Mieszko, her estimated birth date range indicates the probability that she was born from a later marriage.] Ziemomysl & his wife had three children:, Adelajda was the daughter not sister of Mieszko I Prince of Poland, although the primary source on which this is based has not yet been identified.
The name "Mieszko" is just as similar to "Mistiuis" as "Mstivoj".Before Bolesław III Prince of Poland died in 1138, he arranged a division of his lands between his five surviving sons, which marked the dissolution of the centralised Polish state which was to persist until the early 14th century.Poland was reunited when Władysław Prince of Kujavia reasserted control over all the Polish territories, with the exception of Silesia, and was crowned king of Poland as Władysław I in 1320. No direct evidence of this supposed second marriage has yet been found.After Bohemia conquered Krakow and Silesia following the defeat of the Hungarians by Otto I King of Germany at Lechfeld in 955, the Polanian state absorbed the region of Sandomierz and land on the upper Bug and upper Dniester rivers, composed of all these areas and united under its Piast rulers, emerged as a stable political entity in the second half of the 10th century.The conversion of Poland to Christianity in 966 reinforced the unity of the new nation state.In , Prince Mieszko donated his whole country to the papacy, signalling an assertion of his autonomy from the empire (at that time still controlled by the regent Empress Theophano) while at the same time affirming papal pre-eminence, which suggests that they were polygamous marriages.