With my girlfriend last year, I would sometimes have to chat with her mum and it was okay. She was kind of a cranky woman so it was difficult at times but I knew she liked me. Her brother was a drug-taking maniac, but I ended up having a few conversations with him, too, so that was okay, even though I was deathly afraid of him.
A good, close female friend of mine from a couple of years ago introduced me to her mum once when I went interstate. She immediately said, "oh, a face to the name!" She liked me instantly. Her dad, in more X-rated terms, told me I should've had sex with his daughter...but that's a story for another time.
Anyway, the point is that you should just be normal with the parents. I always acted myself and I would sometimes get along better with the parents than my female friends! Light chit-chat usually works. Good luck!