I'm coming to this thread way late but I do have a few things to add.
First, although the vet visit didn't show anything, there is a distinct possibility that the newly-seen aggression could be a result of epilepsy. I've seen this firsthand. An epileptic seizure is not necessarily grand mal (thrashing and drooling and all that). An epileptic seizure, in a dog, can manifest in sudden aggressive bursts and then the dog often goes right back to normal without showing any signs that they even realize what they've done. I had an Australian shepherd that went through this. She would go from normal to attack back to normal in a matter of 30 seconds.
I'm not trying to make excuses, just offering another possibility. BUT whatever the cause, the first thing that needs to be done is to make everyone safe. A dog that bites its owner (or in my opinion, anyone without a high level of provocation) has a poorly developed bite inhibition. Bite inhibition is something dogs learn, usually as puppies. Older dogs who bite can be taught a higher level of bite inhibition but it has to be done consistently and by every single person who interacts with that dog. It's a lot of responsibility - however, I've done it and I've helped others do it.
My 12 year old chow bit me not long after I got her (she was 14 months old). I immediately put her on restriction. No getting on furniture, no treats or petting or attention without earning it, no going through doorways or up stairs in front of me, every bit of food had to be earned (by doing attention or obedience moves or tricks - I literally fed her by hand and she got nothing out of a dish for weeks). Anytime I couldn't watch her, she was confined in an ex-pen (in the living room, so she saw the other dogs being free to interact while she had to stay cooped up). No freedom outdoors - everything on leash - basically she got NOTHING, and I mean NOTHING, for free.
Following an NILIF program is a wonderful step toward teaching a dog that they are not entitled to be the "boss" without resorting to strong-arm tactics. You get inside the head, instead of just forcing the body. But it has to be followed stringently in order to truly work, and too many people fudge it and then complain when their dog continues to boss them around. So if you're going to do it, do it fully. Commit to it.
With the OP's dog, I would also recommend teaching the dog to wear a muzzle. This is a preventative measure that helps provide safety to those who may have to deal with her, but it also teaches the dog that even if they DO try to bite, it's not going to work. Dogs often continue to snap or nip or bite because they've found that it works for them. Nervous? Snap at the person and the person moves away. Feeling crowded or irritated? Nip the person and the person leaves. Biting WORKS because none of us are going to stay there and continue to get bit! By wearing a muzzle, you can do what you need to do and not retreat from her if she does try to snap.
I'm not a big believer in giving my dogs the right to have a lot of personal space. Now, that doesn't mean that I bug my dogs incessantly, or do things that they don't like just because I want to do them. But all of my dogs are taught to accept being handled - EVERY part of their body being handled - and touched and tugged and scratched. Feet are handled. Ears and fur are tugged. I stick my fingers in their mouths and ears and armpits. I open their mouths and check their teeth. I tug lightly on tails. I keep my voice very matter-of-fact and add in a "eht eht" if they start to react, and then praise/reward when they don't. Of course I don't start all of this at once and expect miracles - I do a little bit, and another time a little bit, and gradually work up to more invasive things. I'm not trying to torture my dogs, but I do want them to be acceptable of touching that is not intended to create intense pain. Dogs should be able to handle some twinges or hair pulling or pinches without reacting by biting, but it HAS TO BE TAUGHT.
Now, all this being said, I think it's important that the OP keeps in mind that this is an older dog, with some arthritis that is likely going to make this dog more reactive than the typical dog with arthritis. I wouldn't do anything to aggravate that condition and expect the dog to not react at this point. But all the little things - the NILIF, the confinement, and working on bite inhibition - those should all be done if the dog is going to remain with a family and be at all trustworthy.
CK - your comments on "her brain broke" with your dog - that's kind of what epilepsy is like. The dog's brain doesn't function in a normal, predictable manner and you just can't fix that. Some meds may help for a while, but when a dog seizes they are no longer cognizant of your presence, and biting happens very easily. Sorry to hear that you had a dog that went through such a rough experience, and that your family had to take the brunt of it.
Best of luck to the OP, and you might contact your vet and mention the epilepsy. If you watch your dog, and see times when she suddenly "zones out" for a few seconds, that could be an epileptic seizure. There ARE meds that might help but she needs to be diagnosed first. And there are also meds that help with dogs who have nervous problems. "Doggie downers" might help you with the training, giving her enough control that the training can sink in more easily. I would discuss more with your vet and see what they think.
Melanie and the gang in Alaska