Oh, that's kind of sad for you. Hopefully the delusion will go away and he won't remember it. Remember, it's more confusing and uncomfortable for you than for him. When my stepmother was senile, she use to call me on the phone and we'd get into a fight and she'd hang up on me but then call again in 5 minutes and start the conversation all over again, not realizing we'd just had it. It gave me the opportunity to take a different direction and try to keep the argument from getting angry :-)

The best brother in the world comment was probably true, not at all delusion :-)