Is it legal to drive in Illinois when the penger is drunk? More details below.?

I'm 15, and have my learner's permit. I'm going with my parents to a party tonight and they're driving separately becuase my mom has to leave early. They want me to stay late with my stepdad so I can drive him home if he drinks. If he is my only penger over twenty-one and he is over the legal limit (.08 in Illinois), what will happen if I get pulled over? The only problem I am worried about is the legality of the situation. Can I lose my permit? As I stated above, this party is tonight, so I need an answer soon.