Of course, but let see the specifics in the case above, that is of Alexandra Duff and Maud Duff:
1. They were the daughters of the Princess Royal (at that time)
2. They were the grand-daughters of Edward VII
3. They were great-grand-daughters of Queen Victoria
4. The King's decision to elevate them to princesses was published in the London Gazette, that is, as a statute of
5. Even so, Edward VII decided that they would follow the members of the Royal Family and declared them Princesses
and styled them as Highnesses but not royal
6. Their father, albeit strictly speaking a commoner, was nothing but a commoner. He was an Earl in his own rights, a
great-grandson of William IV (even though out of wedlock) and a great-great-grandson of George III. Last but not
least, he was elevated to the rank of Duke and made 1st Duke of Fife, in the British Peerage.
In essence, notwithstanding the constitutional and legal differences involved, the difference alone between a commoner like Madame Karella and a commoner like the Duke of Fife, Earl of Fife in his own rights, is at least chaotic!
Technically speaking though, insofar as Alexandra and Maud Duff were concerned, you are right, although they were made princesses by Statute of the British Empire, while King Constantine can create no Law*. Last but not least, even older Greek Constitutions never recognized titles outside the Royal Family. Thus, no person could be prince/ss, in Greece without being a member of the Royal Family and the Royal House. To put it differently, even when the monarchy was in effect, if one was not a royal highness he/she could not be a prince.
*As Head of a Royal House, however, he has the prerogative to elevate anyone he likes to princely status, particularly now that he is not bound by any constitution. All I am saying, however, is that elevating these ladies to princely status while their mother remains a commoner is inconsequential and rather unprecedented.