بسم الله الرحمن الرحيم
Ask your mother to read the following dua extensively:
رَبِّ أَنِّي مَسَّنِيَ الضُّرُّ وَأَنتَ أَرْحَمُ الرَّاحِمِينَ .
Your family members should also read the following dua near your mother i.e. one member should recite it and the rest ones should say amen:
أسأل الله العظيم رب العرش العظيم أن يشفيك .
In-shaAllah, she shall be recovered soon.
Allah knows Best!
Darul Uloom Deoband
This answer was collected from the official ifta website of Darul Uloom Deoband in India.