According to Ancestral Roots this Ida or Idonea is not to be confused with her [according to many sources] sister who married Walter Fitz Robert. Instead of being her sister, I have followed the suggestion of Ancestral Roots and made the Ida who married Walter FitzRobert, the daughter of William II Longespee.
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The following is from Ancestral Roots, Line 30-26/27, concerning the Ida Longespee who married Walter Fitz Robert:
Though peerage articles, including CP [Complete Peerage] V 472, have long identified Ida No. 27 [who married Walter Fitz Robert] as a daughter of Gen. 26 [William Longspee & Ela FitzPatrick], it is clear that their daughter Ida actually married (1) Ralph de Somery (dsp) and (2) bef. 1220 William Beauchamp, of Bedford (see 122A-29 (CP XII (1) note g, which says this Ida living 10 Apr 1262). Moriarty (ms. at NEHGS) suggested William Longespee had two daughters Ida, or Idoine, which may be correct. But there appears almost a generation difference between the two Ida's, so the chronicler may have missed a generation and Ida No 27 may be the daughter of William's son William II (122-29). [Ancestral Roots]