Is it your fault if your dogs illness goes undetected and they get sick and die because you haven’t taken him to the vet for years?

We took in my mother-in-laws dog because all she did was keep her in a kennel and only let her out to pee and eat. (She WANTED to give her away, mind you she’s had this dog for 11 years) she hasn’t taken her to vet in over 6 years, she’s 11. We took her to our house and she started getting super skinny and drinking a lot of water and peeing it all out. We took her to the vet and he said she has diabetes and kidney failure, he said that the kidney failure is due to the diabetes. My husband and his brother got into an argument on if his mom is to blame for her not getting the proper care (going to the vet) he says we’re all to blame because we could’ve taken her to the vet, we didn’t know she hasn’t been there in years. I’ve always assumed that’s just what good owners did. the vet said if we would’ve caught it in the early stages that she wouldn’t have had kidney failure and she would’ve just needed insulin for her diabetes. Now, she won’t eat or drink she shakes and lays in one spot all day. We’re probably going to have to put her down depending on what the vet says on Monday. His dad said he wouldn’t take his pet to the vet unless something was wrong with it (they stopped taking my husband and his brothers to the dentist and doctor when they were 12, so I see how they would think that’s okay) but I was wondering if it’s the owners fault if their dog gets sick due to them not going to the vet and preventing it.