Does a wife who had a miscarriage of her 15-week-old stillborn fetus in the restroom, without being able to retrieve it, incur sin or require expiation?

It is understood from the question that your wife did not cause the death of the fetus nor its descent into the toilet, as all of that occurred due to natural actions beyond her will. Furthermore, she was unable to remove the fetus from the toilet. There is no blame upon her for all that happened, for "Allah does not burden a soul beyond its capacity."