Yes, they are. The Hatfield - McCoy feud was a real and infamous family feud. It took place in the late 19th century in the border area between Kentucky and West Virginia. The feud involved disputes over land, livestock, and family pride, which led to years of violence and retaliation between the two families.
It is indeed a true story to an extent. The Hatfields and McCoys' conflict was real, but in its retelling, details might be embellished or simplified. Still, the overall framework comes from actual events.
The feud finally ended when the law got more involved and some family members were arrested. This made the families realize that the fighting had to stop.