I am sorry you are going through this. Has he ever been manipulative before or lied to you about this? If not then maybe he is really telling the truth about things you are seeing and hearing not being there. Can you perhaps video chat with your mom again while your brother is home so you can all discuss what is going on? Maybe he is right, but since he is pretty young and doesn't know everything that is going on, could be good to have your mom involved. Please take care.