Hello Sripad, and welcome. Has your mother discussed any of this with her doctor? Diabetes is a progressive illness, even when we have our sugars in tight control, and it may be that her numbers have crept up slightly and her meds need to be reassessed.
As for getting enough fluids, she can drink anything and everything that isn't loaded with natural or added sugar. Plain water is a good place to start. Coffee, tea, diet drinks. Other beverages like almond milk can be used to make delicious drinks too, and sweeteners can be used if she has a sweet tooth. Cow's milk is likely not a good choice, since it's loaded with lactose.
Sugar-free frozen treats might be an option since the summer heat is troublesome for her. There are low-carb frozen fudge bars, etc. if this is anything she would enjoy. I've gotten hooked on Breyers Carbsmart products, if you need a place to start, and these would still be a good source of fluid.
Keep us posted on how she's doing, and thank you for joining us on her behalf!