"My sister is buying a green backpack."
Translation:Moja siostra kupuje zielony plecak.
you need accusative after kupować, "plecak" is not animated so it has accusative=nominative plecak. Then you need green to match "plecak" so it is also masculine accusative=nominative zielony.
*If you ever heard poles say it different that's because some Poles are confused with the "animation" and some other Poles use it "ironically".