To bury the hatchet

To make peace.
(A hatchet is an axe).

e.g. My cousin and my brother have not spoken to each other because of an argument YEARS ago. My sister has finally made them see sense and persuaded them that it’s time to bury the hatchet.

The idiom comes from the figurative or literal practice of putting away weapons at the end of hostilities among or by Native Americans in the Eastern United States.
Weapons were to be buried or otherwise hidden or put away in time of peace. Europeans first became aware of such a ceremony in 1644.