HOW 148 AND 99 ARE BOTH RIGHT:
99:
The brother was half of his age when he was two. So, when he was two, his brother was 1. 98 years later, He's 100 and the brother is 99.
148:
The brother was half of his age when he was two. However, this could also mean half of his CURRENT age, 100. According to that, his brother was 50 when he was 2. 98 years later, he's 100 and his brother is 148.
Personally, I prefer the answer of 99.