Following the conquest of the temple fort , later known as Jaromarsburg, on Arkona in 1168 by the Danes under King Valdemar I and Bishop Absalon of Roeskilde, the princes of Rügen had to recognise Danish suzerainty. On their accession to power, the Rügen princes were forced to confirm their allegiance to the Danish king and were obliged to provide military support to the Danes. In 1304 Vitslav III of Rügen gained sole lordship over the territory on the death of his brother, Sambor. Because he had no issue at that point in time, there existed the possibility that Rügen's princely house could become extinct. As a result, in 1310 he agreed a contract of inheritance at Ribnitz with the Danish king, Eric VI Menved, his feudal lord. The agreement was that in the event of Vitslav dying without issue, the fiefdom of Rügen would be returned to the Danish crown. At that time, Eric VI Menved was attempting to extend his power in the southern Baltic Sea region in order to reduce the influence of Hanseatic towns like Stralsund. To assist him, in addition to the princes of Rügen, the king also had Prince Henry II of Mecklenburg as a vassal. After the death of Eric VI Menved in 1319, the 1310 treaty lapsed and Vitslav III sought allies in the Pomeranian dukes. On 5 May 1321 dukes Otto I of Pomerania, Vartislav IV of Pomerania-Wolgast and Barnim II of Pomerania-Stettin agreed a mutual inheritance contract with Vitslav III of Rügen. But in an alliance agreement that had been previously concluded on 25 October 1315 between Vartislav IV, the son of Vitslav's sister, Margaret, and Eric's brother, Christopher of Halland, who later became King Christopher II of Denmark, Vartislav IV had already been promised the Rügen fiefdom.
Question: Who was Margaret's brother?
Answer: Vitslav