When Jesus says that he has not come to bring peace to the land (not the ‘earth’ as in most translations) but a sword, he is speaking with the voice of the prophets. For example, Jeremiah 12:12:

Upon all the bare heights in the desert destroyers have come, for the sword of the LORD devours from one end of the land to the other; no flesh has peace.

It is false prophets who declare peace to Israel, but the Lord says that the false prophets and the people of Jerusalem will meet not with peace but with the sword of the Chaldeans:

Then I said: "Ah, Lord GOD, behold, the prophets say to them, ‘You shall not see the sword, nor shall you have famine, but I will give you assured peace in this place.’" And the LORD said to me: "The prophets are prophesying lies in my name. I did not send them, nor did I command them or speak to them. They are prophesying to you a lying vision, worthless divination, and the deceit of their own minds. Therefore thus says the LORD concerning the prophets who prophesy in my name although I did not send them, and who say, ‘Sword and famine shall not come upon this land’: By sword and famine those prophets shall be consumed. And the people to whom they prophesy shall be cast out in the streets of Jerusalem, victims of famine and sword, with none to bury them - them, their wives, their sons, and their daughters. For I will pour out their evil upon them. (Jer. 14:13-16)

Jesus brings the same message: first century Israel should not expect to enjoy a complacent peace and prosperity; what they face is war and destruction. Hagner in his commentary suggests that ‘sword’ is a metaphor for hostility between family members (Hagner, Matthew, 291). This is ridiculous. The prophets did not mean it metaphorically, and undoubtedly many of those who listened to Jesus were to fall to either a zealot or a Roman sword.

Similarly, when he says, ‘I have come to set a man against his father, and a daughter against her mother, and a daughter-in-law against her mother-in-law’, he has in mind another prophetic message of judgment on a corrupt nation. When God smites the land and makes it desolate because of their sins, when the day of punishment and confusion comes, they should put no trust in neighbours and friends: ‘for the son treats the father with contempt, the daughter rises up against her mother, the daughter-in-law against her mother-in-law; a man’s enemies are the men of his own house’ (Mic. 6:13; 7:4-6). This confusion is a sign of God’s judgment on Israel.