The verb “felt” has two complements:
- “slip in under her mosquito netting” which indicates the action that is felt (slip has a standard meaning here: enter the area that's protected by the mosquito net, with a connotation that this is done gently, softly);
- “that sleepwalker whom she had raised” which indicates who performed the action.
The normal word order has the subject before the action: “she felt that sleepwalker whom she had raised slip in under her mosquito netting”. When the subject has a complement that contains a verb, as is the case here, the clause is difficult to parse. So the unusual word order doesn't really make the sentence harder to understand, once you're aware that this inversion is possible.
The word order follows the original: “mientras no sentía deslizarse en el mosquitero aquel sonámbulo que ella había criado”. I don't speak Spanish, so I can't tell whether there's a specific stylistic effect that the translator may have wanted to mimic.
In the French translation, the language is very literary, with a lot of unusual or archaic words and constructions. Such effects are also visible throughout the English translation.
My guess is that the translator deliberately chose this word order in order to make the sentence look more elaborate and old-fashioned, fitting in the general style of the book. Inverting the action and the subject is not standard modern English, but lends an old-fashioned feeling to the sentence.