Duke of Norfolk and Lord Segrave, Earl of Nottingham, 6th Lord Mowbray. The two daughters eventually became the two co-heirs of the Dukedom of Norfolk. The Complete Peerage conflicts with itself on which was the elder daughter. Under Ferrers Vol.V, p357 it says that Isabel was the eldest, but under Norfolk Vol. IX, p. 610 note c, it says that Margaret was probably older as her son inherited the title.

Knight of the Garter
  Earl of Surrey (1483 cr)
  Duke of Norfolk (1514 cr - 1st)

As mentioned before, the Royal ancestor of the Howards was Thomas of Brotherton. Click here for a pop-up showing the full lineage of the Norfolks. Brotherton's arms showed the lions of England with a white label to show that he was a younger son.  Thomas' eventual heir was Thomas de Mowbray, Earl of Nottingham, who was later created Duke of Norfolk. Although he was only descended from Thomas of Brotherton through two heiresses, the then King, Richard II nevertheless allowed him to bear the arms of Brotherton on their own, impaled with the arms of Edward the Confessor.   Normally, Thomas de Mowbray would have borne his own paternal arms of Mowbray, quartered with the arms of these heiresses.  Bearing the arms of Brotherton alone could imply that Mowbray was a direct descendant of Thomas de Brotherton, through the male line (which in those patriachal days bore much more weight than descent through the female line).  Not only that, but as Mowbray was also one of the Dukes that was allowed to impale the arms of Edward the Confessor in imitation of the King himself,  his arms, therefore looked very regal indeed. (See under Richard II for information about the arms of Edward the Confessor).

Married BEF DEC 1378 to de Montacute, William
Married 1384 to Mowbray, Thomas, Duke of Norfolk
Child 1: Mowbray, Thomas of Norfolk, Earl of Norfolk, b. 17 SEP 1385
Child 2: Mowbray, John of Norfolk, Duke of Norfolk 2nd, b. 1392
Child 3: de Mowbray, Margaret
Child 4: de Mowbray, Isabel
Child 5: Mowbray, Elizabeth