Cat Urinating outside of the litterbox.

My wife and I are becoming very impatient with our cat. The cat keeps on peeing outside of the litter box. Ever since it was a kitten, it has had problems with defecating in the litter box. It also has a sister but we never have had problems with her. We have two separate litter boxes. A natural pine litter for the box. At the time of her defecating on the floor, I have placed her in the litter box and would make her knead the litter to show her that is where she should be defecating. That didn't work. I have also secluded her in another room with the litter box to train her that there is only one place to defecate. That would work for about a month. I have taken her to the vet to see if there is a medical problem with her and there was no sign of a UTI or anything else. She is also fixed. At this point we are looking to take her to a shelter. We don't want to split up the family, but we don't know what else is there to do.

Do you have Feliway or have you tried a calming agent?  Have you added a 3rd box?  What about trying different litters, have you done that?

How would a calming agent help in this situation? We don't have room for a third litter box. We are on the fourth different litter.

Have you tried Dr Elsey's Cat Attract litter?