Hi Lori,
Roxie had good advice.
At that level of non-care it doesn't seem likely that your sister is able to have a realistic conversation about what is needed for the good, healthy care of 14 indoor cats.
At the very least the cats are going to be developing upper respiratory problems which are hard to treat since they just keep circulating without proper ventilation and enough space that the cats aren't in constant contact with each other. Keeping cats indoors is a good solution to the problems cats get into when they roam but only if the indoor conditions are maintained to a decent extent.
You could point that out to your sister but getting thru will be hard.
You could report her to your county animal control/shelter. They will likely remove the cats if they think the conditions are truly bad. Unfortunately depending on your area the shelter may not have the resources for extensive vet care or unlimited stay before adopting out. A lot of places will try very hard to place the animals but the reality is unless it's a no-kill shelter some healthy, sweet animals will be euthanized.
Any action you try to take without her consent is going to be damaging to your relationship with your sister.
If you can, try to talk to her (sister to sister) about the overall hoarding problem. Maybe there is an area you could help her clean where the cats could have litter boxes and fresh food and water. If any aren't spayed/neutered there may be services in your area offering that for free.
My heart goes out to you Lori. It's bad enough to watch adult family members live in a hoard. It's unbearable when it involves those who need caretaking like children, the elderly or animals.
For yourself read as much as you can to understand, help if you are able and trust your judgment in making the right choices.