As long as the other party agrees that you can trade on your behalf, your transaction does not violate the compliance rules of the exchange, and it is not illegal. It is illegal if it violates the illegal standards stipulated by the exchange.

For example, if you use the asymmetry of funds and information to grab a hat, it must be illegal.

Or illegal trading, multiple accounts are reversed.

And you can't be a futures practitioner.

Normal operation is not illegal. But the best advice is to let your brother make his own deal instead of dealing with money directly. What should you do emotionally if you lose money for him?