Why did Edmund marry her? Well, it was undoubtedly part of big brother John of Gaunt's plan to secure the throne of Castile for himself. Gaunt married Isabel's elder sister, Constance, or Constanza. The two young women were the daughters and heirs of the deposed Castilian king, Pedro 'the Cruel' who had been murdered by his illegitimate half-brother. Said half-brother then nicked the throne.
From Edmund's point of view this was not a particularly good deal. He and his wife were required to sign over their rights to Castile to John of Gaunt and Constanza - I hope for a consideration. Apart from that Isabel hadn't much but the clothes she stood up in and a few jewels.
Isabel is buried at King's Langley - when her tomb was investigated by curious Victorians in the 19th century she was found to be quite a small lady, estimated 4' 8" in height. If the Chronicles are to believed she - er - liked a good time. It has been suggested that the Chroniclers - churchmen to a person - may have been hostile to her because she favoured the Lollards. This seems improbable, although she did have at least one known Lollard as an executor.
(The Lollards, for anyone who doesn't know, were a sort of early Protestant, though the range of opinions represented by the term is very wide. Orthodox clerics hated them with a passion.)
Isabel had three children, Edward, Constance and Richard, of whom more anon. She is also supposed to have had a lover, King Richard's half-brother, Sir John Holland, Earl of Huntingdon.
If true, this says little for her taste in men, as he was a distinctly nasty piece of work, involved in at least two murders.
When Isabel died in 1393 she left her jewels to King Richard, with a request that he provide an annuity for her younger son, Richard. This the King did, and on a relatively generous basis compared to the value of the bequest. Richard of Conisbrough (often known as Richard of York in his own time, but better known by his birthplace because of the danger of mixing him up with his more famous son) never had any land of his own, and this annuity remained his principal source of income. In royal family terms, he was a pauper.