Microsoft 365 features that help users manage their subscriptions, account settings, and billing information.
Unfortunately you would have to cancel the subscription in your brother's account, or I should say, he should cancel it, then request a refund if this was made within the last 30 days. Then you go sign up for the plan under your account. There's no supported pathway to do a simple switch.
To cancel, Please go to https://account.live.com
Sign into his Microsoft account. Then go to Services. Click on Manage for the subscription plan you want to cancel.
To request refund, go to https://support.microsoft.com/contactus, Type "Billing" and press Enter. On the next screen, click "Contact support" at the bottom of the page, then choose "Microsoft 365" > "Manage my subscription". You’ll then see the option to chat with a Microsoft support agent who has access to your account to help review the issue.
You can reference this detail step by step instruction with screenshots I posted in this thread: https://learn.microsoft.com/en-us/answers/c/2714817