General Notes

John FitzAlan; feudal Baron of Oswestry; one of the magnates opposed to King John; married 1st Isabel d'Aubigny, sister and coheir of Hugh, 5th Earl of Arundel, and died 1240, being with her ancestor of the FitzAlan Earls of Arundel.  [Burke's Peerage, p. 1985]

John Fitz Alan; married 1st Isabel d'Aubigny and had issue; married 2nd Hawise de Blancminster (ie. Whitchurch, Salop) and died 1240.  [Burke's Peerage, p. 2098]

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John Fitz-Alan took up arms with the other barons temp. John, but, upon the accession of King Henry [1216], having had letters of safe conduct to come in and make his peace, he had livery of the lands of his inheritance, upon paying, however, a fine of 10,000 marks. This feudal lord m. 1st, Isabel, 2nd dau. of William de Albini, Earl of Arundel, and sister and co-heir of Hugh, last earl of that family, by whom he had a son, John, his successor. He m. 2ndly, Haws de Blancminster, and dying in 1239, was s. by his son, John Fitz-Alan. [Sir Bernard Burke, Dormant, Abeyant, Forfeited, and Extinct Peerages, Burke's Peerage, Ltd., London, 1883, p. 200, Fitz-Alan, Earls of Arundel, Barons Maltravers]

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Burke's Peerage, p. 2098, on Lineage of FitzAlan:

The d'Aubigny male line died out by 1243, whereupon the huge family estates were parcelled out between the last d'Aubigny Earl of Arundel's sisters.  Isabel, the second eldest, was wife of John Fitz Alan, who through her came into possession of Arundel Castle but, perhaps significantly, did not style himself Earl of Arundel and was not so referred to by third parties.  A contributory factor here seems to have been the longevity of the last d'Aubigny Earl of Arundel's widow, who survived her husband almost forty years and who may in some sense therefore have been regarded as Countess of Arundel in her own right.

Note: I assume the d'Aubigny widow who survived her husband almost 40 years was wife of Hugh d'Aubigny, 5th Earl of Arundel, brother of Isabel.