Even though I suppose all of you have found the solution to this small riddle by now, here it is.
The question that would save you from certain death is:

Which way would your brother recommend to me?

Explanation:

Since one of the twins always lies and his brother always tells the truth, logic demands that if you ask for the way the respective brother would recommend...

the twin who tells the truth will truthfully tell you what his brother would say (= the way to certain death), and

the twin who always lies will also lie about his brother's (originally truthful) answer, so that he will also recommend the way to certain death.

As a consquence, doing the opposite of what either brother tells you will save your life.