I just found out that my nephew (my brother's son) is in department of children and family services. His mother hasn't informed us about this matter (think she's trying to hide it), Anyhow, is there a way I can find out if this is true or not? And, could I be granted temporary custody since he's my nephew?

You'd have to inquire with the department of jobs and family services. It might be that the child is in protective custody or such where the agency might determine it to be in the child's best interest not to have any family contact or potential for the mother to get to the son. If your brother has any parental rights, he might be able to find out more. I don't know if they would discuss the matter with you, but they can at least tell you that they can not due to privacy... etc, but it won't hurt to inquire from them.