There's no any evidence Alexander was a son of Helmond and Sophie.

Alexander Lüneburg (* 1240 ; † 13 July 1302 [ 1 ] in Lübeck ) was a German councilor and mayor of the Hanseatic city of Lübeck.

Life
Alexander Lüneburg came to Lübeck from Livonia . He became the first councillor and mayor of Lübeck for the patrician family of Lüneburg in the city. He was probably elected to the council in 1293 and was treasurer in 1297 and 1298. Also in 1297 he was a co-signer of a loan that the Bremen archbishop Giselbert von Brunkhorst took out in Lübeck. In 1302 he was appointed mayor by the council. He was buried in St. Catherine's Church , where his coat of arms tombstone with inscription is located. [ 2 ] His house was at Johannisstraße 11. He was married to Elisabeth von Morneweg, who after his death married the later mayor Albert von Bardewik . [ 3

https://de.wikipedia.org/wiki/Alexander_L%C3%BCneburg_(Politiker,_1240)

https://de-academic.com/dic.nsf/dewiki/2238901

Borgmester i Lübeck Alexander (Sander) von Lüneburg, 1240 - 1302
MyHeritage släktträd
Född
Alexander (Sander) von Lüneburg föddes år 1240, i födelseort.
Maka
Alexander gifte sig med Elisabeth von Lüneburg (född von Parchim) i vigselplats. Elisabeth föddes cirka 1265, i födelseort. De hade 4 barn: Sabine Von Bockholt (född von Lüneburg) och 3 andra barn.
Personuppgifter
Alexander levde i adress. Hans yrke var Yrke.
Död
Alexander avled den dag månad 1302, vid 62 års ålder i döds plats. Han begravdes i gravplats.