She won't stop! Advice?

So I'm having a problem with Rin, my one year old cat. Before I tell you what though, some background information first. Rin was born in my house on April 7, 2011. When she and her siblings were five weeks of age I started to litter train them. However, their mother continuously pulled them out so that it would end up on the floor instead of the litter box. Circumstances led me to give Mama cat away (to this day I still miss her). Shortly after I found homes for the other siblings, deciding to keep the older sibling from the first litter and Rin. There for a while, I managed to get Rin to use the litter box. She used it without fail, no matter what type of litter it was. Even when I brought in another cat off the street, a female Birman I've named Kiera she used it. I had Kiera a short time though, just a few months before I gave her to my sister. During that month, one of the other cats I've been caring for (he was abandoned last year by his owners) fell severely ill. It was during this time while I was caring for him that Rin started to pick a particular spots in my living room to go poop, and it's always near the front door. When Patch, the abandoned cat became well again to go back outside, I thought my trouble with Rin was over. It wasn't. She continued to do it. My sister brought Kiera back, but that didn't cause Rin to stop. More information. I have three cats in this house and three dogs. Rin was raised with her brother Smoke who is one year older than she is and is neutered, and with the three dogs. Patch lives outside, unable to come in for very long because of the fights he has with Smoke, his son (complicated I know), and with Kiera. Because I have three cats inside the house, I also have three litter boxes. I've used cat attractant to lure the cats to the litter box, and I even use Odoban and EWWWW! Pet stain remover from PetSmart. I've even used white vinegar with two parts water. The litter boxes are cleaned every two days, a step up from once a week. I'm honestly at my wits end. I've tried scolding, showing her the litter boxes, even locking her up in a pet carrier. I've spoken to Rin's vet and even to the employees at PetSmart. They're all stumped, and I am frustrated. I do not want to get rid of Rin, preferring to keep her and Smoke. Kiera and Patch have a possible home lined up, but I do not think that will stop her from pooping on the floor. Is there anything I can do to keep her from pooping on the floor and to use her litter box all the time?