Translation:Please help my younger brother.
Both is fine. There are many phrases in Chinese that you will realise, may only need one of the two words to express the same meaning. Like 怎 in 怎么. It's just that in modern-day Chinese, the 2-word form of these phrases are more commonly used, while the 1-word form of these phrases almost automatically give it an 'ancient-style'. 帮 is however an exception where both are pretty modern-day. I would feel 帮 is the much more casual one of the two. I think 助 also adds to the tone.