What does jefferson say the colonists should now consider their “british brethren”?

The phrase "British brethren" was a way of remembering the colonists' attempts to appeal to the good will of the British without obtaining in return the goodwill of the British Empire. By the time Tomas Jefferson and all the colonists had decided to sign the Declaration of Independence, they had already tried to negotiate peacefully, and to express the needs and ideals of the colonies, but since it could not be achieved in that way, they had to make it clear that war that came was not their first intention, but the last option they had left to the 13 Colonies.