Take a look at the sentence again:
Filled with holiday spirit, Ximena bought a Christmas present
not only for her
boyfriend Andre but also
for his obnoxious younger brother Pierre.
When you use not only ... but also, you must have two equal grammatical items. After not only, notice that you have a prepositional phrase, for her boyfriend Andre. And after but also, you have a second prepositional phrase, for his obnoxious younger brother Pierre. The structure is thus parallel.