We do not know ... because the palace has not released the list.
There was some speculation that Princess Ariane of the Netherlands and Prince Emmanuel of Belgium would be in the bridal party only because the share Prince Guillaume as a godfather. Crown Princess Victoria included some of her godchildren in her bridal party, but not every bride does this.
Dutch royal reporter Marc van der Linden reported earlier today that Princess Ariane will not be in the bridal party. One assumes he got the word from the Prince of Orange's office.
Countess Stéphanie has a few nieces and nephews who may be included, as well as the Crown Prince's two nephews. Prince Gabriel and Prince Noah of Nassau. Guillaume's first cousin, Prince Jean of Nassau also could be in the wedding party.
No word if Prince Emmanuel will be in the wedding. More details will be released toward the end of the week.