In view of the age of your grandma, i would like to suggest as follows that should be done in consultation with your family physicians and without stopping the medicines or prescriptions given by the doctor.
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Simple movements - suksham vyayama which is comfortable and easy to be done as per the stamina or capacity.
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Sectional Breathing - Abdominal breathing, thoracic breathing and clavicular breathing.
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Naadi Shuddhi Pranayama
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Brahmari Pranayama
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Chin and Vayu Mudra for 15 minutes in morning, after noon and evening in any pose even during tv serials or talking
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Yoga Nidra
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Om Meditation for 15-20 minutes
Remember no exertion be made during any of the above practices. The Patanjali Yoga sutra says Asanam Sthiram Sukham - means - yogic posture is that which is comfortable and stable. So do not go out of comfort zone. Never use the theory of 'no pain no gain'.
Though you have asked about the yogic practices for diabetes, I would recommend to focus on diet and lifestyle to manage diabetes. Diabetic diet should be individualised based on the nutritional status of the patient, so, diet chart should be prepared by some nutritionalist or dietitian. Keep weight under control too.
If you have any question, please feel free to share here, i will be happy to answer.