First time babysitting job vs. Volleyball?

Someone wants me to babysit, and they just moved in the neighborhood therefore they a looking for someone to babysit. And they want me too. But the day they want me to babysit I have a volleyball tournament at Pen Sate. It is a 3 day tournament and she wants me to baby sit the first day from four to eleven. I don't want to loose all future jobs with the family but I already made a commitment to volleyball. What should I do?