Who was sleeping in the second chamber in "Macbeth"? Why did Shakesphere include that information in the play?In Act II, who was sleeping in the...

According to Lady Macbeth, Donalbain (Duncan's son) was sleeping in the second chamber in Act II, and the second person with him is assumed to be his brother Malcolm.  After killing the King, Macbeth told Lady Macbeth,

"There's one did laugh in's sleep, and one cried
'Murder!'
That they did wake each other: I stood and heard them:
But they did say their prayers, and address'd them
Again to sleep."

This is important because this is the point where Macbeth realizes he can no longer say "Amen."  He has gone past the point of forgiveness and can no longer pray. 
These two characters (sons of Duncan) are important because after they flee, the blame immediately goes on them and Macbeth is next in line for king.