Cat Not Using Litter Box

Question
I have a 7 year old female cat. A little bit over a month ago, she started going outside of the litter box and using the kitchen floor instead. It is always in the kitchen, sometimes near the fridge or near the stove or sink. It is always a different spot. I just clean the whole floor to make sure I get everything up. I have tried putting down lemon juice to keep her away from the spots but that didn't work. I cleaned to litter box after every time it was used to make more "inviting." It is a covered box so I removed the top just so she can see better. None if these things have worked and it's getting worst.
Two months ago, I gave her a new larger litter box, and she started using it immediately. About 2 weeks after that, she pooped on the kitchen floor. I didn't think much of it so I just cleaned it up and moved on. In the past she hasn't used the litter box when she didn't like something that was going on like when I rearranged my room or painted, things like that. The only difference is that she only did it 3 times at the most. For some reason she has been doing it more frequently and has almost completely stopped using the litter box. I will be attending college in the fall and my mom is going to take care of her while I am away, but she has threatened me that I need to figure out why she is doing this or she is going to get rid of her.
There are 2 other cats in the house, a 4 year old male and a 18 year old female. These are not my cats. They belong to another family member who lives a floor above us. My cat does not roam around like they do and stays on our floor, but the other 2 come and go as they please. They have their own litter box in the basement but occasionally the male cat will use my cat's litter box. He has been doing it for a while but she didn't seem to have a problem with it. My cat gets along with him fine but not with the oldest. They give each other that death stare every time they see each other, but they have not had any fights. As far as possible stressful conditions, we have not done anything major to this house in years. The last thing I did was add a desk to my room and she sleeps under it almost every night. I have been using the same litter since she was a kitten so I don't think she dislikes the litter and the same thing with the food. She has always eaten Purina and I upgraded to Purina One when that came out.  Do you have any advice on getting my cat back into the litter box?

Answer
Hi
you have not mentioned if your cat is spayed or not. if not then this would definitely be the first thing to do as it reduces a cats urge to have to territory scent mark.

i suspect that your cat can smell the other cats and so feels the need to scent mark. using their faeces is a way of making both a visual and scent mark and is known as middening.

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You will need to tray a period of litter training again using the confinement method mentioned on the page above. this helps the cat to settle down and get used to the litter tray again.

Also lots of toys and distractions help to keep the cats attention away from  other cats etc.

To be honest with you, having so many cats in one building is never a good idea especially if they are not brought up together. Also having litter trays which can be used by other cats is not a good idea either. unfortunately your home setup is rather difficult to ensure that this behavior does not reoccur from time to time. You may have to reconsider the home arrangements, which I know will be difficult but sometimes there are just no alternatives.

However try the suggestions i have made and hopefully this will help your cat.

best wishes Kate