Cannabis is an extremely safe herb and has never caused an overdose death. You can increase the dose without any fear of hurting your mother. If her pain or other symptoms are not being adequately managed it would be okay to increase the dose slowly and observe for improvement. You may also try sublingual tinctures, inhaled cannabis, edibles or topicals. CBD and THC are both helpful and should be ingested in a whole plant preparation that contains other cannabinoids, terpenes and compounds that will act to synergystically increase the effectiveness of the treatment.