Is it more of a priority to care for a sick mother than to teach Quran to children on Sunday, knowing that the sister works and teaches Quran on other days of the week?

The mother's right is great, and honoring her is one of the most obligatory duties, while disobeying her is one of the greatest sins, especially during her weakness and illness. Serving and caring for the mother is obligatory upon all her children, either by providing someone to serve her or by doing it themselves. It is not permissible for the sister to refuse to care for her mother during her turn, under the pretext of being busy with teaching the Quran, because honoring the mother is an individual obligation upon her, whereas teaching the Quran is not an individual obligation, and paradise lies beneath the mother's feet.