A tricky situation

I love a boy and we are having a serious relationship for almost 2 years. We have all the plans to get married but this guy is not yet settled career wise. few weeks back we went for a small picnic where i invited my friends and he also invited some of his cousins and his elder brother. His brother is a good looking guy and working on a high position with a renowned company. During the picnic he showed some interest in me as my boyfriend had not told me that i am his girlfriend . he only intoduced me as a friend. But the problem is that i also got attracted towards his brother , we exchanged numbers and we call each other sometimes that my boyfriend is not aware of. i like this new guy in my life and also that he is well setteld in life, is interested in me too. I am not able to understand that how should i tell this to my boyfriend. or should i carry on with my old relationship and forget about his brother.