The profile for Geoffrey states he is "Brother of Sister of Gilbert de Brionne, uncertain parentage." Wouldn't that mean Geoffrey is also a brother of Gilbert de Brionne? However, his father Richard I, 'the Fearless', Duke of Normandy, doesn't seem to have had a child called Gilbert de Brionne.
Medlands https://fmg.ac/Projects/MedLands/NORMANDY%20NOBILITY.htm#_Toc38956176 states, "Orderic Vitalis records that "Gislebertus comes Brionnæ nepos Ricardi ducis Normannorum" married "neptem suam" to “Baldrico Teutonico”, ... ]."
This translate to , Gislebert [Gilbert], Count of Brionne, nephew of Richard Duke of the Normans, gave his niece in marriage to Baudric the Teuton...
Medlands https://fmg.ac/Projects/MedLands/normacre.htm#GeoffreyBrionnedied1015B states, "GEOFFROY de Brionne, illegitimate son of RICHARD I Duke of Normandy .. Geoffroy & his wife had [two] children:
1. GILBERT de Brionne "Crespin" ([979/1000]-murdered [Mar] [1040]).
2. [--- .] m ---. One child:
a) daughter . m BALDRIC"
1) The first Medlands quote states Gilbert is a nephew of Richard, rather than his grandson. I haven't found a nephew of Richard named Gilbert. Was there one, or is this an error?
Both the first and second quote support the concept that a Gilbert had a niece that married Baldric.
2) Could the mother of this niece be the unknown sister? If that is correct, this unknown sister is not a daughter of Richard and a sister of Geoffrey. Wouldn't she have to be a daughter of Geoffrey?
3) Some unsourced sites I looked at, state Alix de Brionne is the daughter of Geoffroy de Brionne and Eu de Clare.. Any thoughts on that?