The very first thing I would do it to have a urinalysis performed on Miley. Even though there is likely a behavior component to it, there may be an underlying medical issue at play as well and if it is not identified and corrected, you will never get the behavior under control. There are some medical issues that are precipitated by stress or anxiety and it certainly sounds like that is occurring. The condition called feline idiopathic cystitis (FIC) can occur intermittently in cats with higher levels of anxiety and the stress or anxiety may not even be something we recognize, but having to share living quarters with other cats, siblings or not, can be one of them. FIC can cause inflammation of the bladder which may or may not result in visible blood in the urine, but it is usually visible microscopically. This can lead to discomfort and some cats going through it will spray or otherwise eliminate in inappropriate areas. It is thought that they can develop an aversion to their litter box as they associate the litter box with pain. The problem can wax and wane on it's own and present very intermittently.
Once a medical condition is eliminated, or if she is found to be free of disease, then you can focus on getting her back to the litterbox. The goal is to make the litter boxes the most attractive place in the house and the other spots less attractive. The general rule of thumb is to have one more litter pan than you have cats. Since there are three cats, you should have at least 4 litter pans in the house if not more. You can have 2 of them side by side if need be. Some cats develop a preference of one for stool and one for urine. Maybe try one covered and one uncovered to find her preference. Clumping litters seem to be preferred by most cats because it is soft. It also allows you to keep it cleaner as there shouldn't be a wet bottom. You may need to experiment with different litters, different types of boxes (short sided, tall sided, etc) and different amount of litter in the boxes. You must scoop daily! There is also a litter box additive called Cat Attract® which works very well. PRECIOUS CAT LITTERS LINK You may have already tried these, but they are suggestions that have helped many of my clients. Litter pan hygiene is key. This is to help with the problem of the inappropriate urine that may be due to a substrate preference. The going on the bed or matress is usually a marking behavior and more a sign of anxiety.
When dealing with anxiety in a multi cat house, it is best to try to identify the source of the anxiety. In Miley's case, it is being forced to share her living space with the others if she doesn't like it. If the cats do not like to live with one another we have to strive to provide an environment where all cats are able to find a place where they feel "safe". That begins by creating an environment of "plenty." There should be plenty of litter boxes, food bowls, climbing towers, toys and resting areas in multiple locations. All the litter boxes and food bowls cannot be clumped all in one place because that forces the cats together, something they don't want to do.
If that is not possible due to the layout of your home, another option is to create a time sharing plan. One cat is out in the house for a bit, then put into a room and other is then allowed out. I would especially practice separation at night. In some cases we just use a screen door on a bedroom to keep the cat separate, they can see each other but not interact, this can be very helpful.
Next, you want to make the other spots unattractive or unavailable, confinement is one way. Maybe keep her from the places it is occurring if possible, especially when you are not home, unless that is where the litter pans are. Enzyme products are needed to break down the odor causing components of the urine. There are a few products that work very well to do that: Nature's Miracle® (link here), Anti-icky poo® (link here) or Zero odor® (link here) Deterrents are needed after that. Carpet runners (with plastic spikes) turned upside down work great for large areas. There are also "scat mats" which give off a small electrical charge. These are not harmful to her. Another option is an indoor invisible fence to keep her out of certain areas of the house. They are very effective. LINK HERE
One thing I would suggest is a product called Feliway®. LINK HERE Feliway is a synthetic pheromone that mimics the natural chemicals that a cat secretes. It creates a comforting, reassuring feeling that has a calming effect. It has been shown to decrease urine marking and spraying within 30 days with a 95% success rate. It is also calming to cats in stressful situations such as transport, hospitalization, veterinarian visits, boarding, new environments, pets or people. Feliway® is a product that can be sprayed or used as a room diffuser. It can be purchased through veterinarians and pet stores. I would definitely put a few of these around the house.
There are drugs that can be tried, but I would try the other things first. Even if you have to resort to drug therapy for a while, these are changes that should be incorporated at the same time. Anti-anxiety meds such as Prozac have been shown to effectively reduce urine spraying and marking.
This is a hard subject to cover adequately in a few paragraphs. I am including a link that may give you a few more ideas. Work with your veterinarian who should have experience with this also as this is a very common problem, but can be solved in many cases especially since she is still a young cat. I am also including links to a website that offer suggestions to provide environmental enrichment for indoor cats. This has become a very important subject as we have now identified that many cat behavior and medical disorders stem from the fact that they are confined indoors and forced to share territory with other cats, which is not how cats naturally choose to live. This "stress" can lead to physical and behavioral problems.
Feline house soiling
OSU Indoor Pet Initiative
I hope this is helpful. Please let me know if you have ANY other questions. My goal is to give you 100% satisfaction and if you are not yet satisfied, please reply so I can clarify for you.