Ben M. Angel notes: A large number of the existing online profiles for Guillaume seem to refer to "The Origins of Some Anglo-Norman Families":

William of Arques has been the subject of an exhaustive study by Professor D. C. Douglas in the introduction to his edition "The Domesday Monacharum of Christ Church Canterbury", where full references are given to the authorities; and it is unnecessary to go over the ground again. Briefly he held Folkestone, Kent, and was the son of Godfrey Vicomte of Arques. The identity of the tenant of Folkstone is established by the fact that it passed to Nigel de Monville who had married his daughter and coheiress Emma. He must not be confused with William of Arques, a monk of Moleme who was a counsellor of Robert Curthose, and still less with William, count of Arques, the uncle of William the Conqueror. The ruined castle of Arques-la-Bataille is well known.

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I've tried to "go over that ground again" and seek out D.C. Douglas' "Domesday Monacharum of Christ Church Canterbury," but it isn't available (for free) online. It seems like the passages related to Guillaume would be vital in determining his participation in the Conquest and his relationship with Osbern, who is presumed to be son of Guillaume d'Arques and Beatrice de Bolbec.

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The Foundation for Medieval Genealogy's Medlands project has an entry for Guillaume (son of Godfroi, married to Beatrix), but it shows that he had only two daughters (son Osbern is not listed). From the page on Normandy Nobility:

http://fmg.ac/Projects/MedLands/NORMAN%20NOBILITY.htm#_Toc254886763

GODEFROI Giffard, 1059, Vicomte d'Arques, married --- de Rouen, daughter of GOZELIN Vicomte de Rouen & his wife Emmeline ---.

Godefroi & his wife had [four] children:

i) GUILLAUME d'Arques .

Guillaume de Jumièges names Guillaume d'Arques as son of Godefroi[527].

"Guillelmus et Gislebertus filii Godefredi Archarum vicecomitis" donated land in Montvilla to Sainte-Trinité de Rouen dated 1059[528].

Vicomte d'Arques. Lord of Folkestone[529].

A charter dated 1080 relates that "Gozelinus vicecomes de Archis…cum coniuge sua et filiis" founded Sainte-Trinité de Rouen and donated property, and that "Willelmus de Archis heres defuncti…avi sui Gozelini" confirmed the donation[530].

m BEATRIX Malet, daughter of ---. She is named in Domesday Descendants[531] as the mother of Emma d'Arques but the primary source on which this is based has not yet been identified.

Guillaume & his wife had two children:

(a) MATHILDE d'Arques .

Guillaume de Jumièges names Mathilde as daughter of Guillaume d'Arques and wife of "Guillaume de Tancarville le Camérier", and adds that they were parents of one son Rabel[532].

m GUILLAUME de Tancarville, son of RAOUL de Tancarville & his wife Avicia --- (-1129).

(b) EMMA d'Arques (-after 1140).

“Manasses Gisnensis comes et Emma uxor eius…filia Willielmi de Arras” founded Redlingfield priory by charter dated 1120, witnessed by “Widonis fratris mei, Rosæ filiæ meæ”[533]. The primary source which confirms her first marriage has not yet been identified. The Historia Comitum Ghisnensium names "Emmam filiam Roberti camerarii de Tancarvilla in Normannia, viduam Odonis de Folkestane in Anglia" as wife of "Manasses"[534], which appears to be incorrect. "Manasses Gisnensium comes et Emma comitissa" granted the administration of the church of Saint-Léonard to Saint-Bertin by charter dated 1129[535].

According to Domesday Descendants she became a nun at Saint-Leonard de Guines after the death of her second husband[536].

m firstly NELE de Muneville (-1103). Lord of Folkestone.

m secondly (before 1106) MANASSES Comte de Guines, son of BAUDOUIN Comte de Guines & his wife Adela [Christina] [of Holland] (-Ardres 1137).