The following post to soc.genealogy.medieval gives a little info about Agnes:
From: Rosie Bevan (rbevan@paradise.net.nz)
Subject: Re: Domesday Descendants corrections: Harcourt & Brus
Newsgroups: soc.genealogy.medieval
Date: 2002-06-08 16:41:02 PST
Cris
Many thanks and appreciation for the trouble you have gone to to give us the synopsis of the Blakely article.
In a nutshell, primary evidence from charters firmly places Juetta as mother of Peter de Brus and Isabel de Brus. Juetta also had a daughter by Roger de Flamville called Agnes, who married William de Percy, Lord of Kildale, Yorks. d. bef 1203, who was a tenant of the Brus family in Ormesby. Agnes married secondly John Birkin and made a grant to Gisborough Priory "for the good of the soul of both my husbands". Agnes' son, Walter de Percy, as heir to his uncle Hugh de Flamville, still living in 1210 [Pipe Roll 11 John, p. 123], who died without issue, confirmed a gift his mother had made to Basedale Priory and a gift his uncle Hugh de Flamville made to Rievaulx Abbey.[VCH Yorkshire North Riding, v.2, p.250]. Walter de Percy was heir to his uncle of a moiety of Flamville, Friton and Holthorpe which indicates that Roger and Juetta probably had another daughter. However, regarding the Arches inheritance, Juetta's son Peter de Brus, was her male heir after the death of Hugh de Flamville. Peter's son, Peter de Brus II was recorded as holding the 7 fees in 1224-30. [Clay, 'Early Yorkshire Families' p.2]