The short answer is: any amount of money is ‘worth’ investing.
$10k at 6% for 40 years is $100k. That might let you retire a couple years early. Conventional wisdom, among people, let’s say 40+ years old, is going to focus on retirement, wishing they’d started saving earlier, and the incredible power of 40 years compound interest.
At 19, though? You’ve probably got college and the potential for student debt coming up. Your first car. The down payment on a house. All of those things can be considered “investment,” too. They might have much better benefit to you, both in the short term and the long term. Or, if your BIL is ready to drop $15k as ‘pocket money,’ maybe you have enough family support that none of those things will be a concern. Hell, maybe you have enough family support that working a job from which to retire isn’t even a concern.
This is a marshmallow problem. Do you want to buy a car, take a fantastic trip somewhere, or just gamble like a big shot right now, or would you rather have less college debt, buy a nicer house, sooner, or retire earlier? Nobody else can tell you what you’ll enjoy more.
Maybe, like his Fuhrer before him, he can experience just a few moments of regret and dread before that death in office. Maybe not a whole day, but at least something.