Am I sinful for waking my younger brother from his sleep so he can sleep with my mother? And is that considered tormenting the child?

Whether or not it harms the child is determined by specialists. If what you mentioned does not harm the child, then you must obey your mother, and you will be rewarded for it. However, if it does cause harm, then you are not permitted to obey her, for obedience is only in what is right, and there is no obedience in sin. If you do what I commanded you, and it causes harm, then you are sinful.