hose sister is Hotspur's wife--has married Glendower's daughter. In other words, there now exists an alliance by marriage between Wales and Northumberland.
The historical account of this line of action in the play is that Glendower was living "peaceably enough" under English rule during Richard's reign but that he became "restive under Henry's increasingly centralized administration," and finally was antagonized when Henry refused to guarantee Glendower's right of inheritance of lands that had been forcibly seized by Lord Reginald Grey of Ruthin (Burke 71). Glendower proclaimed Wales his kingdom, "gathered troops, sacked Ruthin, and ravaged the border Marches," eventually making "plans for a free Welsh Parliament and a university, and wrote to the kings of Scotland and France as one reigning monarch to another" (Burke 71). Shakespeare's 1HIV does not deal in any detail with Glendower except with respect to the fact of his alliance with the Percys and Mortimer against the king. But it appears that Wales was in rebellion against Henry well before the Percys and Mortimer "threw in" with Glendower.
In Shakespeare's play, Mortimer's affiliation with Glendower and Hotspur's defiant defense of Mortimer's motives in a confrontation with Henry (I.iii) is fundamentally a coup, whatever the timing of the alliance. That is because Henry's legitimate hold on the English throne is tenuous; Richard II had named Mortimer his successor. Indirectly, Glendower is positioned as a potential king maker in 1HIV, a position that has some moral legitimacy in the face of Henry's dour insistence on his kingly dignity and warnings against the Percys not to "tread upon my patience" (IHIV I.iii). The Percys, who now regret having helped what Hotspur calls "this proud king" to replace Richard on the throne, join Glendower, Mortimer, and Scotland in rebellion for that reason. Thus the political action of the play proceeds. The rebellion of Wales (Mortimer plus G...