1. Boys may be expressing their affection for you in a lighthearted and humorous way, trying to bring your relationship closer while maintaining friendly boundaries\ The word "sisters" may only be his habitual address, and does not directly represent gender confusion.
2. He may feel very comfortable getting along with you, like a friend, so he chose such a friendly address instead of the traditional one that pursuers would use. This reflects his trust in you and his comfort in your relationship.
3. You can also proactively communicate with him and ask him the reason for calling you this way, to see if he is aware that this may cause misunderstandings for you. Through honest communication, we can better understand each other's thoughts and feelings, and thus decide how to continue developing this relationship.