If they're like family, I'd simply tell them what you've told us. Just tell them that you think they're wonderful, and you appreciate the amazing opportunity they've given you, but that the monotony of the actual task is starting to get to you, and that you really need some variety when you come to work every day. Maybe tell them that as a lawyer, you'd get to analyze situations and how best to attack them and win the case, which is a mental challenge you'd appreciate. But filing, no matter how great the environment, isn't giving you the challenge you need, and it's absolutely nothing personal, but you feel like it's time to find something else.
I think they'll understand. Heck, they may even try to find something around there that would suit your personality better. But likely, at the very least, they'll hate to see you go, but ultimately understand and wish you well.