Parents (both 90) refuse to listen to children about safety when aide leaves. Both have walkers and need assistance. They live alone. Advice?

They are 90, but you don't mention them as having dementia. If they are able to remember camera system and how to disable it then at least one of them is like NOT suffering from dementia. So this then falls into the realm of their own choice, unless I am mistaken.
If I am right, and there isn't a dementia diagnosis, you are going to have to understand that they are alone, vulnerable to fall because of their age, and that this eventually WILL happen. There is at least two of them, and with 911 easy to remember, able to be posted, I think you may be down to that for now.
You understand this is a disaster waiting to happen. You can sit them down and gently explain that these devices are for their safety and their good, and that realistically people of this age DO fall and WILL fall. But I am not certain what else you are able to do but make house as cleaned out, trip proof stair free as you are able, with all needed things easily reached.
I am 78. I recognize that falls now mean that something is more like to break than not. With that recognition the way I move in the world is different, always using bannisters, not carrying things down stairs, no more ladders, and etc. BUT....things do eventually come to this place.
I hope others have better ideas for you to try.

My dad lived alone until he was 96, lived in a three-story house, and used a walker. He also loved having throw rugs around to keep the "good" carpet clean. He was a disaster waiting to happen but miraculously he never fell. I had stair lifters installed so he could get up and down the stairs easily. They had remote controls so he could send them to the bottom or top of the stairs. He used them like dumbwaiters to "carry" stuff up and down for him while he continued to walk up and down. I did have someone do a cutout in the tub so he could just walk in and not step over the edge. Otherwise I just waited for the fall. If you aren't dealing with dementia then they are allowed to live as they want, even though it makes us crazy. My suggestion is just to do your best to do small things that are as unobtrusive as possible in their lives and then let the rest go. No one needs to watch them overnight and the camera is really an invasion of their privacy. Check in by phone each day, make certain both of them can use the phone to contact you in case of emergency, and then let it go. As long as they are capable of making decisions they are allowed to make bad decisions. It's still their life.

Step back. Their choices, their consequences. I have found the hard way that i cannot “make” anyone do what they should do. All I got was headaches and worries. So I stepped back. Unfortunately, you have to wait for something bad to happen. I am sorry... and wish you well.

If your parents don’t have dementia, take a step back and be thankful you don’t have to be their carers. I would give anything to be in this situation. Don’t worry ahead of time. It accomplishes nothing. They have a right to their privacy.

I agree with the 3 posts currently here. In this situation, I don’t think there is much you can do (unfortunately) except accept that they have made this choice. That suggestion is not always easy to hear, but I think it applies in this case. We all only can do what we can do.

Regarding physical therapy coming to the house, that is almost always a good idea. They are experts and can do so many things to try to help prevent falls. Falls usually happen at some point anyway, but physical therapy can help delay it as long as possible. It was very smart of your parents to have therapy come out and they fortunate. Also, if the therapists do not feel that your parents are benefiting from the therapy, the therapy will be stopped. Medical insurance will not pay for therapy once improvement from that therapy stops / no gains are seen by the therapists. Especially with this pandemic when agencies are running very short staffed and they have people waiting to be seen that could benefit from the therapy.

Regarding the phone not being answered: is it possible that you and your brother are just driving your parents nuts with continued questions and suggestions about their health and safety when you call them? Is this why they are no longer answering the phone? You love them and are concerned and don’t want anything bad to happen. Believe me, I get it. But if all the concerned comments, questions and suggestions are now causing them to not answer the phone, maybe it is better to ease up a bit so maybe they will start answering the phone again.

Best of luck to you; I know it’s not easy.

If you installed a camera system without their permission, then shame on you for a gross breach of their privacy.

If their doctors haven't certified them as incompetent, then you and your brother need to step back. Sure, try to got them to wear Life Alert pendants, but otherwise respect their rights to live their lives.

Yes, the "crisis" day will come, but no amount of what you're doing will prevent it from happening. Just back off, have some contingency plans with your brother, and let them have their independence as long as they can enjoy it.

As difficult it is to step back, you need to take a breath and do so. A lovely 87 year old said to me "there are such things as helicopter children. They take their jobs very seriously and at times they lose sight of the fact their parents have made it this far. It is best to step back, be their child again and treasure them. Parents know that they are coming to final days, dont remind them of this every time you talk or visit. End of life is scary. Relax, enjoy their company treat them as parents and friends.

Elders that don’t have dementia are free to make their own decisions, even those we may disagree with. You’re in the position that many here are, waiting for an event that will force change. Please don’t ruin your relationship with your parents by trying to force them to accept what they aren’t willing to do. They are clinging to their independence and life as they’ve known it. And there is something admirable in that, even though it’s frustrating. Hopefully they’ll accept life alert buttons to wear. I wish you well in this

Would they be willing to wear one of those Life Alert type buttons?
If so, get the kind worn on the wrist. More often than not, a person will remove the type worn as a necklace when they shower and go to bed.

Loss of independence and privacy is very tough for many seniors. It is their right and decision to do it their way, just be sure they understand the realities of fall risk and the need to remove risk factors for fall risk management. If they will heed professional advisors, Dr.,Nurse, P.T. it is less....painful.
Many times it is easier to hear from a pro, our family dynamics/issues can be a factor in listening to common sense aging issues. If people are stubborn they may have to live with their decisions. We all fall... eventually. I like the wrist fall/emergency device as well, peace of mind for loved ones and the service will contact a family member if needed.Not driving, that is a big one too!