Successive deaths had now made Philip and Joanna heirs of Spain, as well as of Burgundy, Flanders, and the Empire; the Archduke was already betrothing his infant son, Charles, the future King of Castile, to a French princess, and his open negotiations for the formation of a league against Ferdinand to assert Joanna's right to assume the crown of Castile on the death of her mother Isabella, who was in failing health, had fairly frightened Ferdinand, who knew not whom to trust; for Castilians generally disliked him, and were ready to acclaim Joanna and her foreign husband on the first opportunity--Joanna herself was unstable, violently jealous of her husband, and with strange notions as regarded religion. She would not go to Spain alone, and Philip was determined not to go except on his own terms, and at his own time, and Margaret, living in close contact with the inharmonious pair, struggled bravely to reconcile the clashing interests that surrounded her. There was a talk of leaving her regent of Flanders in the absence of her brother in Spain, and against this Ferdinand's agents were instructed to work secretly; although Margaret lost no opportunity of professing to the ambassador her attachment to Spanish interests. From several remarks in Fuensalida's letters to Ferdinand it is, however, evident that her desire was less to rule Flanders than to enjoy the care of the infants whom her brother and sister-in-law were to leave behind. But even this natural desire was opposed by the Spaniards; apparently because the Princess was looked upon as being too ready to follow her brother's lead. Writing in March 1501 of Philip's dissolute life and his disaffection towards Spain, Fuensalida says: 'I am loath to say how much Madam Margaret's good-nature encourages this, for she simply follows her brother's fancies in all things.'