As an aside and for a bit of fun as it doesn't directly relate to heraldry, I have posted a couple of photographs of Marie Louise, decked out in her finery for the Coronation of 1937 (HM King George VI). The unusual aspect is HH's train. Instead of the more usual affair with gold lace and ermine trimming which the British Royal Family usually wear on such occasions, Marie Louise wore her train with a plain fur trim.
Although there is evidence that she wore the same train to all four coronations she attended (1902, 1911, 1937 and 1953), the latter two were not Marie Louise trying to be different from the rest of the Royal Family, it was because she WAS different!
As they were raised in England as members of the British Royal Family, Marie Louise and her sister Helena Victoria were required to relinquish their father's Germanic title of Princess of Schleswig-Holstein as a result of the First World War and anti-German feelings in the country. In fact, the two Princesses seem to have been the forerunners of the Royal Family in this respect.
The Court Circular published on 20th June 1917 that His Majesty was "....pleased to direct that Royal Warrants shall be prepared declaring that Their Highnesses Princess Helena Victoria and Princess Marie Louise of Schleswig-Holstein shall henceforth be styled Their Highnesses Princess Helena Victoria and Princess Marie Louise respectively, and ... he has also been pleased to direct that Royal Warrants shall be prepared declaring their present styles and making such corresponding alteration in their arms as may be necessary." However, no trace has been found of any such warrant in the London Gazette or in the Warrant Books at the National Archives; nor are any recorded in the College of Arms.
The rest of the Royal Family was covered in the first instance by the proclamations of July and November of 1917, and individual proclamations/royal warrants were made for replacement titles. But not for Helena Victoria or Marie Louise. They could therefore, at best, be described as "Princesses of Nowhere".