I think your BIL gave you both good advice and bad advice.
The good advice, as others have said, is that you need to have a significant fraction of your portfolio in equities.
The bad advice is to buy an S&P 500 fund. I have never thought these funds were great strategy and IMO the arguments against them are getting stronger.
Thing #1: They are a sector bet: large cap US stocks. What we see in history is that sectors wax and wane. A passive portfolio does not make sector bets.
Thing #2: I think there is a tenable argument that the S&P is in a sort or mini-bubble. A majority of naive investors like your BIL have believed the indexing arguments (really arguments for passive investing) and have piled into the S&P. So it is probably overvalued relative to the 60% of the world stocks that have not benefited from their naivete.
Thing #3: Towards the end of every bull market, professionals start to talk about stock held in "weak hands." IOW, people who have bought but will not have the courage to ride out the next correction. Remember the story that JP Morgan got out of the market in 1928 because his shoe shine boy started giving him stock tips? True or not, it's an issue worth considering. So in this bull market, the concern about weak hands increases when talking about the S&P simply because that is where the naive dollars have been going
In ten years or more, most of these things are washed away by time but I still think they argue for not concentrating an equity portfolio in any S&P fund right now. Just buy a total US market fund or, better, total world market fund. Then commit the Copilot's Checklist to memory:
Sit down, shut up, and hang on.
If I wanted to make sector bets, which I do not, I would try to assemble a portfolio that included the world market specifically sans US large caps.