It sounds like a frustrating situation, and VAT issues can be tricky with Airbnb. Since your brother is using the same VAT number without issues, it’s possible that the system has flagged the number for duplicate use, which might explain why it isn’t accepting it for your account.
Here are a few steps that might help:
Contact Airbnb Support: Explain the situation to Airbnb’s support team directly. They may need to manually resolve the issue or clarify if the VAT number can only be linked to one account.
VAT Number Registration: Double-check that the VAT number is registered correctly and updated with your local tax authority. Even if your brother’s account works, there might be a registration issue affecting yours.