FMG seems to follow the Complete Peerage Vol 12 (part 1), pages 492-492 in making Emma the second wife of Ranulph I de Warenne and not the mother of his children. Their mother is Beatrice, probably a niece of Gunnor or Gunnora the wife/mistress of Richard I of Normandy.
However some more recent research, suggests that Ranulph should actually be father and son with the same name, with Beatrice as the wife of Ranulph (1) and Emma the wife of Ranulph (2), and that this fits in better with the dates they are mentioned.
In this second scenario Emma is the mother of William I de Warenne, and his brother who now becomes Ranulph (III), however her parents are still unknown.
There is a bit of discussion about this here
http://www.rootsweb.ancestry.com/~medieval/gunnor.htm