Is she peeing on the bathroom rugs? Cats are often attracted to the rubber backing for some reason. They are also attracted to bleach, so freshly bleached clothes are often peed on. If those are things she tends to pee on, I would put the bathroom rugs up (over the side of the tub or shower curtain) and put the clothes in a drawer out of her reach. Make sure you clean the areas where she peed with an enzyme cleaner designed for cat urine, like Nature's Miracle. Otherwise, she'll be attracted back there by the smell.
What sort of tests did your vet run? Most UTIs aren't true infections, especially in young cats. Did they do a urinalysis to rule out crystals and to check for blood?
Is it possible that your kitty was frightened or attacked while using the litter box? A household with four cats should ideally have five litterboxes. If there is only one, I would definitely add more in quiet areas away from appliances and household traffic. You might also try a different litter in one of the boxes. There are some cat litters designed to attract cats back to the litter box, like CatAttract. I haven't tried it personally, but it might be worth it. If you are using a covered litter box, you could try uncovering it.
Hopefully, some of this will work for you. Good luck.