Without knowing what problems the brother has there’s no way for any of us to say.
Even knowing exactly what problems he has, about the most specific thing an expert could say is about like this: “Everyone is at small risk of producing mentally ill offspring. Your risk is slightly higher than average.”
It seems to me that an answer like that doesn’t really bring you much closer to knowing “What should I do?” How lucky do you feel and how much risk is too much risk? No way for us to know what you think of these things.