Can I marry my cousin brother's daughter

Assuming you both are Hindus. No, you cannot marry you cousin's daughter as you are "sapinda" of each other. As per the hindu marriage act section 3 (f)(i)"Sapinda relationship" with reference to any person extends as far as the third generation  (inclusive) in the line of ascent through the mother, and the fifth (inclusive) in the line of ascent through the father, the line being traced upwards in each case from the person concerned, who is to be counted as the first generation; (ii) two persons are said to be "sapinda" of each other if one is a lineal ascendant of the other within the limits of sapinda relationship, or if they have a common lineal ascendant who is within the limits of sapinda relationship with reference to each of them.