19-year-old brother tells 22-year-old sister to invest in gold, sister demands he pay her back after she loses $1000: 'She sold everything and locked in a $1,000 loss. After she sold, the price went back up.'

He didn't tell her not to sell at a loss. 

One of the first things you learn about investing is that you shouldn't sell your investments the second they drop a little bit. You're supposed to hang onto them for long enough to be able to see them grow, and if you're someone who is not thinking in the long-term, you might not be ready to invest. Before investing, you need to get into a financial position where a $1000 emergency does not put you in the hole. If you have three to six months of cash savings, you have enough wiggle room to start investing without being forced to sell your investments when the going gets tough. 

If you're a regular Joe, you're probably just going to invest in things you think are a safe bet. If you are a self-identified trader, you're probably more willing to take on riskier investments that you believe will grow a lot in a short amount of time. I probably wouldn't take financial advice from a day trader, just because I don't have the time or energy to constantly monitor my investments. I'd rather just stick my money in a retirement account or the stock market and be done with it, because I know that I'll be in a good place if I can max out those accounts eventually. Still, some people really need financial advice because they don't even know the first steps of investing.