Female dogs reaction to cat--Maltese westie mix

Question
Recently I've come home for college and brought my kitten home with me. We have two dogs at home--a large male dog, mixed breed, completely uninterested in the cat, generally shy and afraid of other animals, and a maltese-westie mix, four years old, who is very interested in the cat. We are not sure if the dog was spayed properly, because every once in a while, seemingly in timed intervals, she will have little accident's in the house regardless of how often she is taken out--it seems like some kind of mating marker brought on by a certain "heat". However, since corrective surgery is very dangerous, we just live with it. Who knows what it really is--this is just our assumption.
Since bringing the cat home, we've had mixed reactions with the female. The cat is good natured, outgoing and friendly, cautious though VERY interested in both our dogs, particularly the smaller female. The dog, however, has had mixed reactions to the cat. She seems interested, but has a sort of hungry look in her eyes. The dog is mostly my sister's, and when it was just me home (of my siblings I am the first home, my sister came the day afterwards), I supervised them together, and disciplined the dog for any negative, aggressive or hunting behavior.
Now that my sister has come home, the dog's behavior has worsened. My sister has a sort of deluded sense of the dog's good nature--this is a very nice dog, great with people, but perhaps she has something against cats or other animals, especially given she is part westie, a hunting dog. She chases the cat, provoked or unprovoked. The cat sort of trots up to the dog sometimes, which is ordinary cat initiation/play behavior. Perhaps the dog is mistaking this for something else--some kind of potentially aggressive behavior. However, at this point, given the kind of hungry, focused look in her eye and her unprovoked chasing behavior, we decided to keep them in separate rooms. Now, this morning, I took the cat out of my room to go get some breakfast, and came back to find that the female dog peed on my bed where the cat and I sleep.
I think that this might be an indication of the dog's territorial aggression towards the cat. I have tried to give many different details of their interaction to give you a full picture. Sorry if this is a bit convoluted.
How can we improve the situation? At this point I'm ready to pack up and go home. However, I would like to enjoy the rest of my vacation without locking the cat up in my room and spending the entire break here with her.

Answer
Am I understanding that you are home for the holidays and not living in this situation? It may just be that the dog is not completely house trained. she may be having urinary tract infections, or she can actually have some retained tissue which causes her to have "heats" though you don't often see inappropriate urination due to that. If it's a temporary situation, I'd get with your sister and take turns keeping your animals up, so everyone gets some free time. And I'd close the door so the dog can't get on your bed. I would be concerned that the terrier mix is having a predatory reaction to the cat (thinks the cat needs hunting) and that can be deadly, so I'd definitely be very careful. Sandy Case BFA MEd CPDT www.positivelycanine.com