Question: Ivar the Boneless was the brother of which 9th century Viking chieftain?

full_context: Document 1: Great Heathen Army: The Great Viking Army or Great Danish Army, known by the Anglo-Saxons as the Great Heathen Army (OE: "mycel hæþen here"), was a coalition of Norse warriors, originating from Denmark (and likely also from Sweden and Norway) who came together under a unified command to invade the four Anglo-Saxon kingdoms that constituted England in AD 865.
Document 2: Uí Ímair: The Uí (h)Ímair ] , or Dynasty of Ivar, was a royal Norse dynasty which ruled much of the Irish Sea region, the Kingdom of Dublin, the western coast of Scotland, including the Hebrides and some part of Northern England, from the mid 9th century.
Document 3: Ivar the Boneless: Ivar the Boneless (Old Norse: "Ívarr hinn Beinlausi" ; ) was a Viking leader and a commander who invaded what is now England. According to the "Tale of Ragnar Lodbrok", he was the son of Ragnar Lodbrok and Aslaug. His brothers included Björn Ironside, Halfdan Ragnarsson, Hvitserk, Sigurd Snake-in-the-Eye and Ubba.
Document 4: Bagsecg: Bagsecg (also known as Bægsecg or Bagsec) (died 8 January 871) was a Viking leader commonly referred to as a king. In 870 or 871 he led The Great Summer Army to England. His forces joined those of the Great Danish Army which had already overrun much of England. He and Halfdan Ragnarsson became the leaders of a joint invasion of the Kingdom of Wessex. He was killed at the Battle of Ashdown, fighting a West Saxon Army led by King Ethelred and his younger brother, the future Alfred the Great.
Document 5: The Strongbow Saga: The Strongbow Saga is a historical fiction series by Judson Roberts. The Saga tells the story of Halfdan Hroriksson, a young Viking trying to avenge his brother's murder. The series weaves the story of the fictional Halfdan into the lives of actual historical figures of the period, including famous Viking leaders Ragnar Lodbrok, Ivar the Boneless, Bjorn Ironside, and Hastein, as well as Frankish leaders King Charles the Bald and Count Robert the Strong. Actual historical events form a backdrop for parts of the story, including a Danish attack up the Seine River in the year 845 which was reported in several contemporary Frankish sources, including the Annals of St. Bertin.
Document 6: Early Scandinavian Dublin: The First Viking Age in Ireland began in 795, when Vikings began carrying out hit-and-run raids on Gaelic Irish coastal settlements. Over the following decades the raiding parties became bigger and better organized; inland settlements were targeted as well as coastal ones; and the raiders built naval encampments known as longphorts to allow them to remain in Ireland throughout the winter. In the mid 9th century, Viking leader Turgeis or "Thorgest" founded a stronghold at Dublin, plundered Leinster and Meath, and raided other parts of Ireland. He was killed by the High King, Máel Sechnaill mac Máele Ruanaid, which was followed by several Irish victories against the Vikings and the seizure of Dublin in 849. Shortly after, a new group of Vikings known as the Dubgaill ("dark foreigners") came to Ireland and clashed with the earlier Viking settlers, now called the Finngaill ("fair foreigners"). The wavering fortunes of these three groups and their shifting alliances, together with the shortcomings of contemporary records and the inaccuracy of later accounts, make this period one of the most complicated and least understood in the fledgling city's history. In 853 a Viking warlord called Amlaíb (, possibly Olaf the White) arrived and made himself king of Dublin. He ruled along with his brothers Ímar ("Ívarr", possibly Ivar the Boneless) and Auisle ("Ásl"). For the next fifteen years or so, they used Dublin as their base for a series of campaigns against Irish kingdoms. During these conflicts they briefly allied themselves with several Irish kings. The Dublin Vikings also carried out a number of raids in Britain at this time. The deaths of Ivar (c.873) and Olaf (c.874) were followed by internecine conflict among the Vikings. Although intermittent warfare between the Vikings and the Irish continued, these inner conflicts weakened the Viking colonies and made it easier for the Irish to unite against them. In 902, Cerball mac Muirecáin, king of Leinster, and Máel Findia mac Flannacáin, king of Brega, launched a two-pronged attack on Dublin and drove the Vikings from the city. However, in 914 the Vikings now known as the Uí Ímair (House of Ivar) would return to Ireland, marking the beginning of the Second Viking Age.
Document 7: Halfdan Ragnarsson: Halfdan Ragnarsson (Old Norse: "Hálfdan" ; or "Healfdene"; Old Irish: "Albann" ; died 877) was a Viking leader and a commander of the Great Heathen Army which invaded the Anglo-Saxon kingdoms of England, starting in 865. According to the tradition recorded in the Norse sagas he was one of the sons of Ragnar Lodbrok, and his brothers included Björn Ironside, Ivar the Boneless, Sigurd Snake-in-the-Eye and Ubba. He was the first Viking King of Northumbria and a pretender to the throne of Kingdom of Dublin. He died at the Battle of Strangford Lough in 877 trying to press his Irish claim.
Document 8: Auisle: Auisle or Óisle (Old Norse: "Ásl "or" Auðgísl" ; died c. 867) was a Viking leader in Ireland and Scotland in the mid-late ninth century. He was the son of the king of Lochlann, identified in the non-contemporary "Fragmentary Annals of Ireland" as Gofraid, and brother of Amlaíb Conung and Ímar, the latter of whom founded the Uí Ímair dynasty, and whose descendants would go on to dominate the Irish Sea region for several centuries. Another Viking leader, Halfdan Ragnarsson, is sometimes considered a brother. The Irish Annals title Auisle, Ímar and Amlaíb "kings of the foreigners". Modern scholars use the title "kings of Dublin" after the Viking settlement which formed the base of their power.
Document 9: Ragnvald Heidumhære: Ragnvald Heidumhære (or Rognvald) was a legendary, possibly historic, petty king or chieftain of Vestfold in what is today Norway in the 9th century, according to "Ynglingatal" and to "Ynglinga saga" in "Heimskringla". He was the son of Ragnar Lodbrok or Olaf Geirstad-Alf. His name "Heiðumhæri" could be translated as "highly honoured"
Document 10: Odda, Ealdorman of Devon: Odda, also known as Oddune, was a ninth-century ealdorman of Devon. He is known for his victory at the Battle of Cynwit in 878, where his West Saxon forces defeated a Viking army led by Ubba, brother of the Viking chiefs Ivar the Boneless and Halfdan Ragnarsson.
Document 11: Ubba: Ubba, also known as Hubba, Ubbe, and Ubbi, was a mid-ninth-century Viking chieftain and one of the commanders of the Great Army, a coalition of Norse warriors that in AD 865 invaded the Anglo-Saxon kingdoms of Northumbria, Mercia, East Anglia and Wessex.
Document 12: Ímar: Ímar (Old Norse: "Ívarr" ; died c. 873) was a Viking leader in Ireland and Scotland in the mid-late ninth century who founded the Uí Ímair dynasty, and whose descendants would go on to dominate the Irish Sea region for several centuries. He was the son of the king of Lochlann, identified in the non-contemporary "Fragmentary Annals of Ireland" as Gofraid. The "Fragmentary Annals" name Auisle and Amlaíb Conung as his brothers. Another Viking leader, Halfdan Ragnarsson, is considered by some scholars to be another brother. The Irish Annals title Amlaíb, Ímar and Auisle "kings of the foreigners". Modern scholars use the title "kings of Dublin" after the Viking settlement which formed the base of their power. Some scholars consider Ímar to be identical to Ivar the Boneless, a Viking commander of the Great Heathen Army named in contemporary English sources who also appears in the Icelandic sagas as a son of the legendary Viking Ragnar Lodbrok.
Document 13: Hastein: Hastein (Icelandic: "Hásteinn") (also recorded as "Anstign", "Haesten", "Hæsten", "Hæstenn" or "Hæsting" and alias "Alsting") was a notable Viking chieftain of the late 9th century who made several raiding voyages.
Document 14: Hvitserk: Hvitserk ("Hvitsärk") "White-Shirt" was one of the sons of the legendary 9th-century Norsemen Ragnar Lothbrok and his wife Aslaug. He is attested to by the Tale of Ragnar's Sons ("Ragnarssona þáttr"). He is not mentioned in any source which mentions Halfdan Ragnarsson, one of the leaders of the Great Heathen Army which invaded the Kingdom of East Anglia in 865, or vice versa which consequently led some scholars to suggest that they are the same individual with "Hvitserk" being only a nickname.
Document 15: Vikings: Vikings (Old English: "wicing"—"pirate", Danish and Bokmål: "vikinger" ; Swedish and Nynorsk: "vikingar" ; Icelandic: "víkingar" , from Old Norse "víkingar "), were Norse seafarers, mainly speaking the Old Norse language, who raided and traded from their Northern European homelands across wide areas of northern, central and eastern Europe, during the late 8th to late 11th centuries. The term is also commonly extended in modern English and other vernaculars to the inhabitants of Viking home communities during what has become known as the Viking Age. This period of Nordic military, mercantile and demographic expansion constitutes an important element in the early medieval history of Scandinavia, Estonia, the British Isles, France, Kievan Rus' and Sicily.
ground_truth: Ubba