There are a couple of ways you can accomplish this. If you insist on using XPath to do it, you need to add the context node to the locator, like this:
WebElement parentElement = childElement.findElement(By.xpath("./.."));
Alternatively, you can use the JavascriptExecutor
interface, which might be slightly more performant. That would look like this:
// NOTE: broken into separate statements for clarity. Could be done as one statement.
JavascriptExecutor executor = (JavascriptExecutor)driver;
WebElement parentElement = (WebElement)executor.executeScript("return arguments[0].parentNode;", childElement);