Re: How to deal with my sempai?
Seems to me that this is more of an internal conflict with you. This other person seems to be allowing you to maintain your teaching role and they aren't stepping on your toes. In my current dojo, there are lower ranked yudansha who teach, but the higher ranked yudansha are more then willing to help if asked upon. Usually you see them off to the side chatting for a sec or two when we pair up.
In my last dojo, it wasn't uncommon for lower ranked students to teach. Brown belts often ran the class because in order to get your black belt, you had to be proficient at teaching. It was a way of being able to learn to teach, but you still had help there if you needed it.
Look at this person as a valuable tool to use if you think you need it.