What would a good brother do?
如果你弄懂了句子成分的知识，这样就不需要再问类似的问题了。
A good brother would do what.
What在这里是宾语。特殊疑问句把需要提问的句子成分提前，打个比方如果这句话的陈述句是A good brother would help you. 根据主语提问，问句就变成Who would help you.
换句话说疑问句的语序要不要改变取决于你针对哪个句子成分提问题。
而从句永远都用陈述语序，只是把代表某个句子成分的关系词提前。
所以回去一开始的那句
A good brother would do what
关系词是what
提前 what a good brother would do.
再说回去who would help you
这句话关系词本来就在句首，所以不用改语序
最后：问问题找根本，问一句话怎么写没有用，因为根据一个语法点我可以瞬间出一百道题目。问题是你刷的过来吗？
