Legally she IS a Princess, but just like Stephanie she will not be called "Princess" during the Nuptial Mass until after the Nuptial Blessing is pronounced.
Stephanie was called "Comtesse" or Stephanie de Lannoy by the Archbishop during the religious ceremony...not Princesse.
In the eyes of the Roman Catholic Church, they are not yet validly married.
It will be the same for Claire.
"Be who God intended you to be, and you will set the world on fire" St. Catherine of Siena
"The arc of the moral universe is long, but it bends toward justice" MLK Jr 1929-1968