The main possibility is that this offer/request comes not from your professor at all, but from a scammer. So you ask your professor directly "I received a message asking for an $80,000 investment, seemingly from you. I suspect it is some scammer impersonating you, so you might want to warn all your contacts". This is very useful if it is indeed a scam, and sends out a very strong message if this was indeed the professor.
The second possibility is that this is indeed your professor. That would be unethical on a massive scale. And what an earth would make him think that you or your sister have $80,000 to invest in his business? And how could he not see that this is a huge red flag? So make sure that you have 100% evidence of this "offer" in case he gets any bad ideas. However, the "scammer" is indeed the much more likely explanation.
Now if you are afraid to go this route, you tell them instead "This sounds good. Can you send me all the details about your business that I need to make an educated decision? Business plan, cost and sales forecasts, and so on. I'll show them to my friend XYZ who runs a business and doing quite well, and if he agrees, then I may consider investing. And my friend might consider investing as well".