My father and mother-in-law made a last living gift to each other. When my father died, she chose the option ¼ in full ownership, ¾ in usufruct. As my hereditary reserve is subject to this usufruct, can I bring an action for reduction?

No. Article 1094-1 of the French Civil Code states in its 1st paragraph that "In the event that a spouse leaves children or descendants, whether or not from the marriage, he or she may dispose in favor of the other spouse either of the property he or she could dispose of in favor of a stranger, or of one-quarter of his or her property in property and the other three-quarters in usufruct, or of all of his or her property in usufruct only".

read the article on donation between spouses