Uncle Is Delusional, Should I Tell Aunt?

Reader Concerned Niece writes,

I recently got a call from my uncle asking me to have lunch. This in and of itself is very unusual. While I love him as part of my family, we are not particularly close. He would not tell me why he wanted to have lunch. A little background, my uncle has always been a bit odd. For example, for the past 15 years he has been working on a mathematical formula to beat the lottery. No winnings to show for it yet, but its a relatively harmless eccentricity, so the family has mostly humored him.