The witches can supposedly see into the future.  This "sight" shows them Macbeth and Banquo's futures.  This is what causes all of the trouble, because Macbeth believes it.  Macbeth is so blinded by greed that he cannot see what he is doing.  The same is true of his wife.  She does not see what she has done until it’s too late, and at that point she loses her sight and enters a sleepwalking fugue state.

You can look at this in two ways; Macbeth as a character, and "Macbeth" as a play. If your teacher wants you to focus on Macbeth as a character, then think about his deeds and desires. He is "blind" to the fact that his actions will cause a great deal of harm, as referenced when he states:Stars, hide your fires;
Let not light see my black and deep desires:
The eye wink at the hand; yet let that be,
Which the eye fears, when it is done, to see.  The first line is a request by Macbeth to not allow others to gain knowledge of his murderous plan/desires. The third and fourth line is a way of saying, “Hey, even if this is wrong and gruesome, I am still going to do it. ”You can think of "the lack of sight" as being figurative. It is not a literal blindness, such as a physical disability. It is a lack of knowledge that is COMPARED to blindness.Be brave. The study of literary elements is just a matter of thinking beyond the text - For example, what connection can you make between what the author wrote and what you experience in your own life/reality? Have you committed an act that you knew was morally wrong, but you still had to do it? Have you ever had someone ask you why you did something wrong, but you cannot “see” the answer?

See the attached website for other references.

How is Sight and Blindness related to Macbeth?

Macbeth is blinded by ambition.  The witches claim to see the future, and it is this view of the future that Macbeth sees.  Lady Macbeth is also blinded by ambition, and she is able to push him along.  Later, she is blinded by guilt and Macbeth is blinded by paranoia.