My nephew isn't allowed to use the toilet? What the hell do I do?

Its not ok that she is doing that. First of all there is no reason at all for her to do what she is doing. There is a deeper meAning to it, ask her why is she afraid? Tell her thats her kid who she is messing up, he needs to learn. I might understand that she has had hard times as a single parent but its not her sons fault. I understand shes your sister but you have to defend your nephew. He cant speak for himself. She is only hurting him, get thru her, make her see that what shes doing is wrong and unfair to that poor kid. I really hope that you will get thru her, and if she needs help get her some. I wish you the best and i hope that if she wont teach him you will at least teach him so he will know at school but get her help. Good luck