Crimes committed: Pooping outside of the litterbox- next to it, a few feet away, and using the nearby carpet as toilet paper.
I'm going to start with the most obvious thing to me - the bum scooching. This is a classic sign of two things: worms and impacted (blocked) anal glands.
Step one is to take a fecal sample in to the vet and get her anal glands checked. Personally, my bet is on impacted glands which can crop up later in life, but checking for worms is easy and should be done just in case.
Throughout her entire life. Consistent and persistent throughout the last year. Poops outside the box every few days or more often. The 'toilet paper' problem within the last year or so.
Curiosity says, has your vet considered constipation for her ongoing issue? Another good reason to take a sample in to the vet. If she's been mildly constipated (not uncommon for kitties fed dry food only) her whole life that would easily explain the issue.
You've got the litter box situation well covered it seems, so no suggestions there.
I am curious about if she's only pooping outside one specific ox, or if it's all of them...
Blue buffalo dry food. Previous foods were purina and felidae. No free feeding as she is a piglet and still gets her brother's food when she can.
Blue isn't a bad food, but the general recommendation here (and for VERY good reasons) is an all wet food diet. Wet food is not more likely to make them fat, won't wreck their teeth, and IS overall healthier for them. If you switch to all wet you might find (if you're lucky) that the pooping out of the box problem magically disappears...although I doubt it as there's most likely a habit that's formed over the course of her life along with the ultimate issue (likely health related, IMO).
I'd suggest picking a grain free wet food or three and feeding only that for a month or two to see what that does for her issues. If the pooping goes away you've got a good answer.
She'll still have to go to the vet to have her glands checked, but it'll save you on the expensive vet foods.
In July 2013 she was diagnosed with suspected inflammatory bowel(IBS) or lymphoma. Long story short we were told an endoscopy was unnecessary since treatment would be the same so she is on prednisolone for life. This medication does not treat nor seem to effect her pooping problem. She is going to get bloodwork re-checked next week. I would like to see her lose a lb or two but even when she was at an ideal weight this has still been a steady problem throughout her life.
It may be related, so I'm asking. What is her main symptom that led to your vet suggesting IBS? Soft stools are the most common, but my oldest kitty Jitzu's is expressed via ejecting the offending food item back the way it came.
Ultimately, there is likely going to need to be some re-training done. That is a process that involves locking her up when you aren't home or able to supervise, and when she is out closely monitoring her behavior so you can make sure only the behaviors you want to see (ie, using the box) occur.
You can get Cat Attract Litter Additive (not sure if that's the right name exactly), and you can add that to whatever litter it is that you're currently using. It's the same cost as a box of litter, but if you're careful it can last MONTHS, you don't need to use much.
....In conclusion ... help? Honestly she is the dumbest cat I have ever had despite the fact I've taught her to give high 5 ...easy to reward when she wants to offer THAT behavior, hah!. Her mother was the pinnacle of instincts matched with great intelligence and her brother is fine so sometimes I'm just stuck thinking 'where did you come from?' Oy!
Lol, I must say I'm a bit sympathetic to this. My youngest kitty Muffin is a very sweet kitty...but not the brightest. We just giggle at his silliness and enjoy his charms. *shrugs*
As for the frequency of cleaning, again the boxes can be totally clean and new and she will still poop outside of them.
...*cough* brand new boxes with brand new litter are a problem I occasionally come across with my pet sitting clients. Adding a new box is a GREAT idea, especially given I usually meet them when they have the added stress of their family leaving that can prompted accidents. That being said, if I am asked I always suggest 'seeding' the brand new pristine box with a deposit or two from the previous one. Spotless boxes are not always an obvious indicator - especially for kittens and those less than genius kitties
As for the feeding...We've tried canned food but it is tough since she goes nuts for it and her brother is a super picky eater- it makes feeding them difficult. Especially since my mom won't follow my guidelines for feeding them. They used to get scheduled meals but since Runt has had this diagnosis the feeding schedule has gotten wonky. Runt will inhale(then vomit) her food if we don't give 3+ small meals a day and her brother will take his sweet time and then drive my mom nuts over which wet foods he will eat, when he wants to eat, etc.
Well, luckily I have a semi-solution, although not a permanent one. Part of retraining your girl will mean confining her for large portions of the day to a room that meets all her needs...if she'll be in her own special room then she can eat her meals and her brother will be left to pick at his to his heart's content.
That being said, I'd suggest a conversation with your mother. One point free feeding proponents have not usually considered is that how much a pet is eating is a very common indicator of health. Skipping one meal isn't cause for concern, but if free fed it can also be very easily missed. Skipping 3-4 meals becomes immediately obvious when you're feeding in meals at set times, and removing the food 20-30 minutes later.
People who feed in meals routinely catch this sort of thing 2-3 meals earlier than free feeders, especially in homes with more than one person, IME. It's easy to assume someone else fed the cat, when the reality is that it's lunch from yesterday! Especially since your two are getting older it's much better for you to know what their eating, when, and how much. That means feeding in monitored meals.
However, what you said about her missing out on something prior to birth is something I've wondered myself. I am not joking when I say this, she is lacking in some areas mentally. She will periodically sit and hold up one front paw. Just an inch or two off the ground. This is something she has done occasionally ever since she was about a year old. There is nothing wrong with her paws. She also has trouble retracting her claws at times, say, when she is playing or after she has stretched up on something(another reason I try to remember to clip them).
Muffin runs into walls, gets stepped on, and regularly gets tricked out of his toys by the other cats. Just like people cats are not all made equal *shrugs*
The paw-thing reminds me of what Jitzu does. She injured her paw about 6 years ago, we took her to the vet and it was a very small hairline fracture. Other than giving her some pain meds (which she liked a little too much, lol) there was nothing to be done. I made such a fuss over her every time she lifted her paw that now she does it as a plow for attention...but she's forgotten which paw she hurt. So she'll sit there, look at me and lift her paw. When I don't respond she switches to the other paw to see if that will work. Very funny
It is also possible she's developing arthritis, but as long as this little personality quirk doesn't extend to limping I wouldn't be too worried.