He was highly probably the son of Richard de la Mare and a sister of Herbert de Castello (Sanders’ Baronies p. 28 and Keats-Rohan’s Domesday Descendants p. 388).

Herbert de Castello was the younger brother and heir to Eutrop de Castello, the heir to their father Helgut de Castello.  Herbert dsp and his heir was Agnes de la Mare.
___________________________________________________________________

However the dates in K-R’s DP and DD seem to stretch the geneartions a bit.  She has:

  Helgut de Castello d. soon after 1086.

  His son Eutrop succeeded Helgut.

  Eutrop was succeeded by his younger brother Herbert I de Castello.

  Herbert I was succeeeeded by his son Herbert II between 1022 and 1168.

  Herbert II d. c. 1189, to be succeeded by his probable gt-niece Agnes who d. 1197.

This is 103 years from the death of Helgut to the death of his grandson Herbert II.  I know my father died 116 years after his paternal grandfather but that was a particularly long interval and my fahter died at the ripe old age of 94, highly unlikely for the 12th century.
___________________________________________________________________