From Wikipedia
Louise is styled as "Lady Louise Windsor",[2] although letters patent issued in 1917 (and still remaining in force today) assign a princely status and the style of Royal Highness to all male-line grandchildren of a monarch.[13] Therefore, all else being equal, Louise would have been styled as Her Royal Highness Princess Louise of Wessex. However, when her parents married, the Queen, via a Buckingham Palace press release, announced that their children would be styled as the children of an earl, rather than as princes or princesses.[14] Thus, court communications refer to her as Lady Louise Windsor.[15]