He probably kept the secret about his brother liking you because he doesn't want to lose you to his brother. That is the ultimate betrayal is to lose a loved one to a family member because if he loses you to his brother he will still have to see you at family functions and it will be a constant reminder if that were to happen and this is why he kept it a secret. The brother on the other hand probably didn't ask you to dance or anything else like that out of respect for his brother. He doesn't want to have a riff with his brother because he likes you. Unless you like the other brother I wouldn't make a big deal out of the brother liking you so that you don't make your relationship suffer because you know this now. As adults we have to make the right choices and not cause drama where there doesn't need to be drama. This is probably why your boyfriend kept it from you don't hold it against him.
It's probably because he doesn't want any hard feelings between him and his brother and he knows that if his brother found out that he may like you it would make him angry. If your boyfriend doesn't know I think I would keep that way there is no need for unwanted drama where there doesn't need to be any. If your boyfriend's brother is not bothering you I would let it go for now but if he makes move on you then I would consider telling your boyfriend about his brother but until that happens it better left alone.