Is it permissible for me to lead my mother in her obligatory prayers when performing the Sunnah (supererogatory) prayers?

It is permissible for someone performing a supererogatory (nafila) prayer to lead someone performing a obligatory (fard) prayer, such as you leading your mother in prayer while she is performing an obligatory prayer. This is because it is permissible for someone performing an obligatory prayer to pray behind someone performing a supererogatory prayer. This is supported by the hadith of Mu'adh ibn Jabal (may Allah be pleased with him), who used to pray the obligatory prayer with the Prophet, may Allah's prayers and peace be upon him, and then return to lead his people in that same prayer, which would be an obligatory prayer for them and a supererogatory prayer for him.