Language / Help Required Teaching Sister Polish [9]

Lately, I've been trying to teach my 3 year old sister polish, and I've been trying to talk to her in Polish as much as possible. But the problem is she'll either ask "What you sayin'?", respond in English if I say "nie" or "tak" (which she often mistakes for "duck"), or she won't understand at all. And also, I would like to teach her how to read and write in Polish eventually, but after she's learned to read and write in English without getting confused between the two. So basically, my questions are 1) Could I just speak to her in Polish often enough that she'll understand what I'm saying sooner or later, 2) Are there any places on the Internet where I could buy children's books in Polish, and 3) At three years, is she even young enough to just "pick up" on a language? Any answers are appreciated! Dziękuję! :D