Your grandparents moved from Kenya to the U.K. after independence. Do you know when they moved? And are your grandparents of Asian descent? The reason for these questions is that it may indicate if they retained British nationality after Kenyan independence and whether or not they arrived in Britain before certain immigration rules changed in 1968 and again in 1973.
Your grandmother naturalised as a British citizen after you were born- did your grandfather also naturalise as British, and if so, when?
In order to show that you're automatically British, you need to show that your mother was a British citizen
or permanent resident when you were born.
Worst case- if you can't show any of the above, you can apply for registration as a British citizen on Form T. All you need to show is that you were born in the United Kingdom and lived in the U.K. until age 10.
https://www.gov.uk/government/publicati ... zen-form-t
Your brother can also register under Form T- or if his father's details are documented on his birth certificate or otherwise, he could also register under form UKF (lower fee than Form T).
https://www.gov.uk/government/publicati ... n-form-ukf
You indicate that you may be Spanish- are you a Spanish citizen?
This is not intended to be legal or professional advice in any jurisdiction.