Am I accountable for anything after I swore that my nephew was in the bathroom, and it turned out he was not?

If the swearer doubted the presence of her nephew in the place and swore to his presence, then this is a prohibited and sinful oath (yamin ghamus) from which one must repent, and for which there is no expiation according to the majority of scholars. However, the Shafi'is mandate expiation, which is feeding ten poor persons or clothing them or freeing a slave; whoever cannot afford this should fast for three days. If the swearer thought and believed in his presence at the time of swearing, then there is nothing upon her, as this is considered a vain oath (laghw al-yamin), which is excused, in accordance with the Almighty's saying: "Allah will not impose blame upon you for what is unintentional in your oaths, but He will impose blame upon you for what your hearts have earned. And Allah is Forgiving and Forbearing."