Is it normal to like your friends brother?

Its not bad. Its natural to like someone. But sometimes it creates problem if he/she is best friends brother /sister. But its not a problem.

Is it against girl code to date your best friends brother?

Under no circumstances should you date your friends brother in secret. Its shady and plain rude to your friend. Getting permission from your friend will make you feel better about yourself and really show you if this relationship is even worth it!