The school only allowed six persons per child, the royal family was already exceeding it:
1 Don Felipe de Borbón y Grecia
2 Doña Letizia Ortiz y Rocasolano
3 Doña Sofía de Borbón y Ortiz
4 Don Juan Carlos de Borbón y Borbón
5 Doña Sofía de Grecia y Dinamarca
6 Sr Jesús Ortiz y Álvarez (grandfather)
7 Sra Ana Togores y Guisasola (step-grandmother)
8 Sra Paloma Rocasolano y Rodríguez (grandmother)
9 Sra Menchú Álvarez y Del Valle (great-grandmother)
I don't think the Ortiz family is "more important" than the other. Note that none of aunts/uncles/cousins/cousines, not from paternal and not from maternal side, were attending. Were Don Juan or Doña Mercedes (the royal great-grandparents) still alive, they would have attended as well. We can be sure about that.