Grandmother being abused by aunty and uncle

So my grandmother (nani) stays with two of my uncle’s families (mama’s families) turn by turn. Both my uncles are married and have their wife and kids with them. The problem is with one of the uncle’s family. We suspect that my uncles wife harasses my nani a lot. And not just verbally, physically. Do note that my nani is in her 70’s now and very fragile. She also is a patient of Alzheimer’s hence can’t remember stuff. Many a times when we videocall my nani (since we live in a different city) we have seen many bruises all over her face! When we asked about it to my uncle he said that it’s due to Her falling over on her own. But we feel it’s something else. We know that it’s a case of DV. But don’t know what we can do about it or prove it. My nani gets a decent amount as monthly pension as she is the spouse of late govt officer. My uncles family takes the advantage of the pension but doesn’t take care of my nani properly. They don’t feed her properly and keep on abusing her a lot which we have witnessed with our own eyes in the past. But the bruises on her face is just too much! Please suggest what action can be taken