Try this,
public class SiblingAndParentInXpath {
@Test
public void testSiblingAndParentInXpath(){
WebDriver driver;
String driverPath = "C:\\geckodriver.exe";
System.setProperty("webdriver.gecko.driver", driverPath);
driver = new FirefoxDriver();
driver.manage().timeouts().implicitlyWait(10, TimeUnit.SECONDS);
driver.get("https://kuknus.com");