I can relate to what your sister does. I do it too. If anyone ever caught me I just make jokes about it that sometimes I enjoy talking to myself. I reminisce or say ideas out loud. If I don't say them out loud a voice in my head won't go away until I say it out loud. I suffer from extreme anxiety and truly believe I have a personality disorder. I seem normal and I have lots of close friends but I'm complicated and they all know it. I go through extreme patterns of feeling extremely high and excited about life to down and depressed with the snap of a finger. I feel like talking out loud or having conversations with myself allows me to understand myself better. I say something stupid to myself and then laugh out loud and make a comment like someone else is there. This is usually during my highs when I have a lot of energy. I went through a lot of trauma alone while young. i didn't have Anyone to go through it with skin depended on myself for my gapoiness. I believe that's the stem of It for me because I know I can depend on myself to listen to me. Hope my insight helps you to try to understand her better