Hi,
Digital purchases like Minecraft Realms are usually not eligible for a refund.
However, since the subscription was purchased on the wrong Microsoft account by mistake, I recommend contacting Microsoft Support as soon as possible. They can review your purchase and determine whether an exception can be made. If a refund is approved, you can then purchase the Realms subscription using the correct Microsoft account.
Note: Since the purchase was made using your brother's Microsoft account, Microsoft Support may ask to verify that account. If so, it is best if your brother contacts Microsoft Support directly.
Go to https://support.microsoft.com/contactus, select Billing and subscriptions, then choose Contact Support.