Is it permissible to return to my fiancée after I swore to treat her as forbidden to me, like my mother and sister, due to the intensity of the situation, knowing that I wish to return to her after a year has passed?

It appears that this wording constitutes ẓihār. Scholars have differed regarding the occurrence of ẓihār from a non-mahram woman, holding two views: that it is valid, or that it is void and of no consequence. Ibn Qudamah stated in Al-Mughni: "Ẓihār from a non-mahram woman is valid, whether absolute or conditional on marriage. So, whenever he marries her, he must not have intercourse with her until he performs expiation." It is also possible that the ruling of ẓihār does not take effect before marriage. This applies if you intended ẓihār; otherwise, there is nothing upon you. Out of piety and precaution, you should perform ẓihār expiation if you intend to marry this woman. We also alert you that breaking off an engagement without a legitimate reason is not appropriate.