How can I help my stepbrother?

My father and his new wife adopted a child. It's actually my step mother's grandson so he is not blood related to me. I have 2 children of my own and really just love kids. I feel so bad for this poor child. He's 3 and still takes a bottle full of juice to bed every night. Because of this most of his teeth are rotten. He has major behavioral issues because they never discipline him. He has no interaction with other kids except when mine visit. Then he bites and hits them. I think of myself as a pretty good parent so I will casually bring up ideas to my parents about the problems with him. My step mother thinks she knows everything just because she is older. I offer to take the child for a weekend so he can do fun things with other kids but the stepmother acts like I wouldnt be able to handle him. My parents are in their late 50's so they don't really have the energy to be raising a young child. I know once he hits his teens he's going to cause major trouble. How can I get my parents to listen to me?