Can you tell where the fluid is coming from? I am guessing that it is the vulva from the rest of your question, and that would indeed be the most likely source. Can you be more specific about the situation in which you find the fluid? Are you seeing a wet spot when she gets up? If so, look at her backend immediately to try to see if it is from the vulva or the anus. Can you actually see the fluid dripping from her? I have also seen dogs that pass a little bit of fluid from their anal glands and then lick, lick, lick at where the fluid landed and the owner finds the big wet spot from licking. Anal gland fluid is pretty smelly, but the area where they are licking may not retain the smell. Can you tell me what was done for the $150 and what medication she was put on?
I think your first instinct that it was urine may be correct. Urinary incontinence is pretty common in spayed female dogs and if the urine is fairly dilute, it may not have much smell or color to it. Luckily, there is medication for this, although it is a good idea to have a urine sample checked first to make sure there is no evidence of infection.
In case your sister didn't go into any details, a stump Pyometra is when the small amount of the uterus that was left after the spay surgery becomes inflamed or infected. I would expect that the discharge would be more like mucus rather than clear watery discharge with this condition. The best way to diagnose this condition is with ultrasound of the abdomen.
Vaginitis could also account for some chronic discharge from the back end. She may be quite inflamed around the vulva in severe cases. Again, more of a mucus discharge would be expected.
Definitely try to pin point where the fluid is coming from if you have not already done so. If you can be certain it is from the vulva, that will help your vet know what type of conditions to check for. Also let them know if you have seen her licking her back end a lot.
It sounds like you are not very happy with your vet at this point and I understand that. Perhaps it is time to try a different clinic. You might try asking any of your friends who have pets which vet they use to find a new place to go. A urine should be checked to help rule out a urinary tract infection and possibly some blood work would be in order, especially if you have noticed any change in her water consumption or if any abnormalities are found on physical exam. Sometimes x-rays may be recommended depending on the results of the above mentioned tests.
If you can be more specific about the situations in which you are finding the fluid, I may be able to help you better.