Perhaps, you could find useful information about consanguinity and
legitimation of children in the Panormia of Ivo of Chartres, Liber VII,
especially Cap 76 ff. "In quo ramusculo consanguinitatis legitima fieri
connubia possint." See, e.g., Migne's Patrologia Latina 161, col 1299ff.
Although your problem is not answered there"expressis verbis" , it seems
that consanguinity was regarded by Ivo and his predecessors exclusively as a
biological , but not as a juristic phenomenon: E.g. "Conjugem ducere sicut
non licet Christiano de SUO sanguine usque ad septimam gradum, sicut etiam
nec licet de consanguinitate uxoris suae conjugem ducere propter carnis
unitatem..." (Cap 79). Consanguinity could be defined by ONE parent, as a
minimum. E.g: "Avus enim et avia TAM EX PATRE QUAM EX MATRE accipiuntur..."
(Cap 90. ex Isidoro of Sev.). As regards Alain Fergent, I think that
bastardy of a daughter HADVISA could not justify a marriage between his
grandchildren, by any way. Probably one of the sources is false.
More later, in a personal letter!
PS: Hadvisa is NOT the same name as Heloisa!