Please help????????????????

My nephew is 5 years old and out of control. He's my brothers son and I love him to death. I have a really small dog and when ever he comes to my house he tortures my dog. We know some times kids play rough with animals, but one day he came over and he had a ink pin in his hand and he stabbed my dog with the pin. I heard the dog crying so I went 2 c what happened, he told me and I asked why he did it and he told me he didn't know. I told his mother what he did only thing that came out of her mouth was"awww ". I'm sure any other parent would have told their child that what they did was wrong, but not her. Hmmm. This past Sunday we all went to my sisters to bring his birthday gifts, since my nephews mother doesn't want us to know where she stays. My mother had got him a card and put money in it, he opened the card and all the money feel on the floor, he wasn't going to pick it up until my mother told him to. He didn't even say thank, at the age of five u might have to stay on ur children about that, but his mother didn't even tell him to say Thank you, most normal mother's would tell their child to say thank you. My sister bought him a hot wheels set, he opened that gift up and said I already have this which is normal for a child to say at that age I guess, his mother said oh yeah he has that, she didn't tell him to say thank you. Every time someone tells him something he'll run to his mom and lean on her and she'll kinda pat him. Should we say something to her? The way she's raising him is not good. And I understand u can't tell someone how to raise their child, but he's out of control. He's slapped his teacher, he throws chairs at school when he doesnt get his way. When he's with us and not around his mother he's an okay child. But when he's around his mother he's a monster. What should we do because we can't stand to be around him which Is sad?