Brother's actual speed ratio = 100: 95 = 20: 19.

Assuming that they reach the finish line at the same time, the speed ratio = (100+5):100 = 21:20 < 20:19.

So the hypothetical brother's speed is slower than the actual one.

So my brother will win.

Or, when the elder brother reached the finish line, the younger brother ran (100+5) ÷ 20×19 = 99.75m <100m.

So my brother ran to the finish line first, and my brother won.

I don't understand, please ask.