My grandmother drinks water and soda excessively and is constantly thirsty, barely eats. Any advice?

Hello everyone! My grandmother is 83 years old and had a stroke about 12 years ago. These days she has huge trouble with short term memory, barely remembers what happened just a little while ago when we talk to her but day-to-day functions she has no problem with. Our latest problem is that she drinks ALL THE TIME. She is capable of drinking up to 8-10l of liquids every day but eat very little, constantly stating that she's not hungry at all, just thirsty all the time. She prefers the sweet drinks but will drink water as well if nothing sweet is available (we try to keep the sweet drinks away as much as possible). She is overweight and isn't losing any weight (though I think if that may be the sweet drinks). She lives with my brother so she's not home alone at nights and we drop by couple times a week. We also take her to regular doctor visits and they never say anything out of the ordinary (she has high blood pressure and takes medication for that and her memory), but lately it's become really spooky. It's like she's obsessed with drinking. She can't think of anything else and if we tell her that she can drink in a little while, she will completely tune us out and just sit in her chair looking at the water. She gets up at night to drink several times. Maybe I'm just over-analyzing things but any feedback would be greatly appreciated. Thank you.