Boyfriend's brother said: Sister-in-law is good, how to reply him?

My boyfriend's brother is called brother-in-law, and he is also a brother. There is another saying. If you get married in the future, then your boyfriend's brother is older than you. You must call your sister-in-law, and you call him brother, so he says sister-in-law is good. You can answer, well, the same thing will do.