Grandma's unhealthy cats, one with a mystery condition?

So my grandmother (my Oma - she's German) has three cats, the mother and two brothers of our two family cats. The are well into adulthood, and some would say senior, but not especially old - 9 or 10 years for the younger ones, not sure for the momma cat. Older, but not into the ancient, declining health years.

Our two cats have been mostly healthy - but every one of her cats has at least one major health problem. All three are incredibly obese, two have diabetes, and one is just a mess. He has something wrong with his kidneys, and tumors or crystals on his lungs and liver? (Remember that these descriptions are all based on things she's told me.) They've seen two vets who weren't sure what to make of this last cat, Mikey's, xrays, and even had one send the scans off to some big vet school that apparently concluded that the masses in his organs are "crystals" and not tumors.

Externally, aside from being incredibly overweight and having regular coughing fits, Mikey still acts like a normal healthy cat. But my Oma has decided that he's going to die, so she'd rather spoil him with unhealthy things than make any effort to get him healthier and possibly prolong his life. She is madly in love with this cat and probably can't deal with the idea of losing him, but I just can't wrap my head around her deciding that a functional cat is on death's door.

I am very well read on cats (a little obsessive about it, honestly), especially health and nutrition. I very strongly believe that most of the health problems our cats face can be attributed to diet, and in the case of my Oma's cats' ailments, there is science to back me up. But she doesn't listen at all anymore.

She lets her cats live off of treats and cheap, filler infested fish based foods. She got the idea in her head at some point that cats eat fish and that's healthiest for them, and so insists on feeding them basically only fish. I've tried to explain to her the health issues that causes, and that cats can't exactly hop into a body of water and snatch up a fish to eat everytime they're hungry, but she's set in her ways.

I think she's getting a little tired of me mentioning it (I only say anything when she brings their health up to me, and I try not to come off pushy or condescending), but she loves these cats and I just can't understand how that translates to only doing things she's decided makes them happy (I think a lot of their preference for salmon is in her head), rather than trying to do what's healthiest for them. It's not like the cats can decide for themselves, you know?

I also tried convincing her to get another opinion on Mikey, maybe get a biopsy done, but she's given up. She's decided that this cat is going to die and refuses to do anything about it.

I just can't understand it. These cats are not mine, I can't just fix things for them on my own. But is there anything I've missed that could help the cats or help my Oma understand, even if its just a little? Has anyone ever dealt with a similar situation to Mikey's, and how did it go?

What about buying some good quality food or treats for them? If she sees them enjoying those foods/treats perhaps she'll agree to continue feeding them. Freeze dried meat treats aren't expensive at all and easy to find, and most cats go nuts for them. 

It does sound very frustrating. I can't see how anything can be done without your Grandma's cooperation though. Is there a relative she trusts who may be able to get through to her? 

I don't know about the mystery condition but obviously obesity is very dangerous and can cause many kinds of health problems. The cats need help with losing weight, probably through wet food and exercise. Maybe you can help enrich their environment with cat trees and other ways to make them more active?

Also, maybe your Grandma would listen to a specialist? There are veterinarian who are also qualified nutrition experts (real ones, not "self-professed"). Maybe talking to one over the phone can help? They can try and assess the diet that your Grandma is feeding and see what changes can be made to help the cats lose weight.

@missmimz She has at least started including freeze dried and single ingredient treats (along with things like Temptations), but still only fish varieties.
My cat is very picky, so I've given my Oma some of the things she's refused to eat. (Top quality, healthy foods and treats.) I don't know how much she tried at first, but I'm pretty sure she's not even trying anything anymore. She's decided they don't like them before she tries them. She did branch out last time we ordered treats and ordered a couple other meats, but I'm not sure she's opened and tried...any of them? She did with the rabbit, because I asked, and they didn't like it, which probably reinforced the idea that they only like salmon.

@Anne Just my mom, but I'm not sure how much luck she'd have, or if she'd be willing to try that hard. She's been with me every time I visit and we talk about it, but I get the feeling she wants to stay out of it rather than try to change her stubborn mom's mind. ^^;; But I can ask.

They do have cat trees, and get to roam my Oma's backyard during the day (nothing to keep them in but being too big to climb the fence), but come to think of it I only ever see a couple of toys laying around that they ignore. Maybe I could get a wand toy and play with them while I'm over there?

I get the feeling my Oma doesn't want to see any more vets. She's still seeing the one locally with some regularity, but she has given up on putting in any extra effort to help her cat. (He has the coughing fits and something showing up on xrays of his lungs - she says if you can't breathe, you can't live, so he's going to die.) But I will definitely look into it still and see if there's a good chance to bring it up.

Part of the problem is that she does think their food is healthy, so she doesn't see that as being a problem she needs to fix. She started getting the freeze dried treats because she would at least acknowledge that all the treats weren't healthy, but she thinks her cheap grocery canned and dry food are healthy food. (She feeds Fancy Feast, so I tried to get her to switch from the Medleys salmon pate to the Classics salmon pate, but she said they wouldn't touch it. I question how much she tried though.) We were talking just the other day and I mentioned something about getting all that expensive healthy food for my cat to try and make sure she was as healthy as possible and wouldn't develop any health problems because of something I was doing (it wasn't a jab at her - I referenced my late kitty's health problems and how they were probably due to her diet). She tried to argue that Angel, the momma cat, is diabetic despite eating healthy, so just eating right won't keep them from developing health problems. (She thinks just that cat eats healthy because Angel doesn't want treats, just dry and wet food.) She doesn't understand that a fish based food that is made for cats might not be that good for them. Like I mentioned above, I've tried healthier grocery store options, I'm not trying to push the top of the line stuff on her.

Maybe if I show her and explain why certain ingredients aren't good for them (especially the diabetics and kidney issues!) she might understand better? That it's not just unhealthy because its cheap, its the ingredients? I think that might be part of the issue, that she thinks I just want expensive top of the line stuff, and doesn't quite get that I actually read labels. But she also might be too annoyed with it to care to listen if I try to explain.