During a bicep curl exercise ? Firstly I would see how many repetitions I can do until volitional fatigue. Then I would get my brother to do the same. Then since we likely do not weight the same I would make it relative to body weight (ideally you want to make it relative to fat free mass, but I won’t have access to these tools to figure that out). So let’s say you did 50 reps at 30lbs that’s a total volume of 1500lbs (50x30), at 180lb body weight. 1500 divided by 180 is 8.3. That’s your strength to weight ratio. Calculate the same for your brother and compare. The reason weight must be included in the calculation is because it would be unfair if you were for example a larger individual with more muscle mass. OR just see who can do more reps until failure.