Err... what if they refuse? How do you know that they are going to be working for this crime family? How do you know what they are going to do before they do it? My suggestion when planning that sort of game is to recognize that the game can change at any point from the players working for X to, the players working against X. You need to have some contingency plans to handle this change.
It's no different than in any other campaign, really. If anything, that extra element of the unknown is what makes these kind of underworld games more intriguing than your standard rescue mission. Even if you go "cheap" and threaten their lives if they refuse, there are still ways for the PCs to give the illusion of getting the job done without their bosses knowing the truth. There's just as much risk of players turning down a chance to do a good deed as there is a bad deed... it's just not as considered as one might think.
Players - good players, mind you - will go along with certain elements so long as they know it's part of the story and they don't feel forced into it. Hell, sometimes they'll go with it even when they are forced if they trust you. I've had players keep secrets from each other for years at a time and they were always welcome to spill their guts anytime they wanted but because they were involved in the secret, they were willing to keep it up. And the players left in the dark were pleasantly surprised when they found out another player was in on the action. It takes the onus of the surprise away from the GM (because players NEVER trust a GM) and brings the players into the action. Allow everything to happen organically and you'd be surprised how it works out.
The trick is knowing your players. If you don't think your players will go for it, don't risk it.