"How many psychologists does it take to change a light bulb?
One. But the light bulb has to want to change."

If your sister believes that the pool store guys know more than us, there's no point..."the light bulb has to want to change."

But if she worries they are BS'ing her, then we can help. Phosphate fighting is the latest expensive scam pool stores are pushing.

First, we need test values posted. If the pool store ran them, then post them. We'll need FC, CC, pH, T/A, CYA, and CH.

What are the dimensions of the pool? Is it round, or oval and how deep? (the water depth, not the wall height.

With those numbers we should be able to tell what's going on. But I expect that, as usual, massive dosing to shock levels with good ol' bleach for several days will fix it.