The brother may not see you as a person in your own right, and / or he may be having problems dealing with his feelings of attraction to you. He is letting you know, you would not be marrying only a person, but a family, a culture, a given way of life. If your boyfriend gives in to his demand to visit him alone, he may or may not always give in.

It may simply be another cultural thing, that they consider it disrespectful for men and women to be together if they are not related. Do you know nothing about his beliefs? At this point, I am somewhat concerned for your safety, as your initiating a break-up may be viewed as shaming. This is not a cultural thing, it's just an angry prideful insecure unenlightened persons thing (the first mr hankster!).