What is the ruling on obeying a mother when she commands one to commit a forbidden act, knowing that not obeying her may lead to a deterioration of her health or even her death?

It is not obligatory to obey the mother if she commands disobedience to Allah. If the matter is not forbidden but involves some doubt, the majority opinion is that obedience to parents is obligatory in it. However, if it is permissible and leads to the father's anger, then it is obligatory to combine obedience to both of them. If that is not possible, the more correct view is to prioritize obedience to the mother. Obedience to her becomes even more emphasized if disobeying her would harm her.