What does Act 1 Scene 5 reveal about Macbeth and Lady Macbeth's relationship?
Act I, Scene 5 of Macbeth reveals that Lady Macbeth and Macbeth are doppelgangers [German for "evil twin"]; that is, they have become doubles of each other.
In their connection in violence with the plot to kill King Duncan, Lady Macbeth becomes more masculine, and like her husband, she invokes the spirits. She calls upon the preternatural world to unsex her.
And fill me, from the crown to the tow, top-full
Of direst cruelty!
This plea of Lady Macbeth asks the spirits to allow her to possess a similar nature as that of her husband, Macbeth. For, he has recently displayed "direst cruelty" himself as he has fought the traitorous Macdonwald, the Thane of Cawdor, who has conspired with the Irish. The brutal Macbeth has "unseamed him from the nave to th' chops" (1.5.22).
So, when Macbeth enters, Lady Macbeth informs her husband that Duncan comes that night to their home, and he can leave all the rest to her:
...you shall put
This night’s great business into my dispatch,
Which shall to all our nights and days to come
Give solely sovereign sway and masterdom. (1.5.79-82)
She now plans things with knives and swords (the murder of the king), in the manner of her brutal husband who slay Macdonwald.