If you are charged with the care and feeding of your cousin's pool in their absence, You really do need a good test kit. Yes, it's not your pool. However, it would be to your and their advantage to be able to cut the ties to the pool store.
Using nothing but a cheap OTO/Phenol red test kit almost invariably ends in what you have described for that pool. Including being taken for more $$ than you (or they) really need to be spending on the pool.
Sure, my little Intex has less than 1/3 the water volume that your cousin's pool does. I am able to keep mine looking really good because I went the expense of a good kit. Once you learn about proper pool water care, you will be glad to have a means of accurate testing.
I go through 1 gallon of bleach every 1-1/2 weeks, and 1 quart of muriatic acid a month in caring for mine. I don't need anything else for my pool, on an ongoing basis. How much do your cousins spend on theirs every time they are home?