Which women in Macbeth have a pivotal role in the play, and how?

The Weird Sisters certainly have a pivotal role in the play.  They ignite Macbeth's ambition by telling him that he will become king, and they plant the seeds of jealousy in him when they tell Banquo that he will father kings.  It seems unlikely, based on Macbeth's initial response to the Sisters' predictions, that he would ever have dreamed of becoming king.  He begins the play as an incredibly loyal subject to his kinsman and...

The Weird Sisters certainly have a pivotal role in the play.  They ignite Macbeth's ambition by telling him that he will become king, and they plant the seeds of jealousy in him when they tell Banquo that he will father kings.  It seems unlikely, based on Macbeth's initial response to the Sisters' predictions, that he would ever have dreamed of becoming king.  He begins the play as an incredibly loyal subject to his kinsman and friend, the current king, Duncan.  However, once the Weird Sisters' first statement, that he will become the Thane of Cawdor, comes true, Macbeth begins his passage down a dark path of betrayal and corruption and tyranny.  Without the Weird Sisters' involvement, it seems probable that Macbeth would have remained loyal to Duncan.

Macbeth's wife, likewise, has a pivotal role.  He tells her, "We will proceed no further in this business" but she urges Macbeth on, persuading him to kill the king, even after he's decided against it.  Without Lady Macbeth's involvement, it seems likely that Macbeth would not have gone through with regicide.