The Solution

The Question You Must Ask:

Which way would your brother tell me is the path to life?  

The Explanation:

The brother who always tells the truth knows that when asked this question his brother would lie and point to the path of destruction. Therefore, when asked the question he would truthfully answer by pointing to the path of destruction.

The brother who always lies would know that his brother would answer the question honestly and direct any travelers to the path of life. However, being the liar that he is, he would point to the path of destruction instead.

So, by asking the question, "Which way would your brother tell me is the path to life?" you get the same answer from either, regardless of which one you asked. They would both point to the path of destruction.

Then you could confidently choose the path that they did not indicate, knowing it would lead you to eternal life.