Chaplain
Think of it this way.
You are taking a university course about turtles. You have an assignment to write a paper about turtles. Coincidentally, your sister Molly also took the same class last year and also wrote a paper about turtles.
You CANNOT use Molly's paper as a source for your paper. That's obviously stupid.
However, you can look at what sources Molly used in her paper, and then use those sources to do your own research for your own paper.
Does that make sense?
(Molly is Wikipedia)