You can choose your friends, but you can't choose your family. You can, however, choose how you interact with your family members. If you've got family members who are codependent, especially those who have drug or alcohol problems, it's natural to want to just get away from them. You have your own family and your own life now, and shouldn't have to continue fixing everyone else's problems. Sometimes, getting away is the only choice you have.
Begin breaking emotional ties. You can see that it's your mother calling for the third time, so don't answer it. The more contact you have with codependent family members, the more connected you are to the situation. Even when you do get roped into the immediate problem, do your best to distance yourself, offering limited advice, if any.
Research job opportunities in other areas. Whether you want to move an hour away or a plane ride away, you won't get too far if you don't have a steady paycheck. Getting a job before you leave has the added benefit of taking a bit out of the sting of leaving. Your family might see this more as a move about your career than their toxic habits.
Create a support network in the new city. Use the Internet to find other moms in the area. On a site like MeetUp.com, you could meet with mothers in general or even sometimes a specific type of mother, such as those who have the same religion or the same age kids. You may even be able to find other moms who have been in a similar situation. At the very least, you'll have someone to meet for coffee and a friend for your kid.
Pack up and move. Refuse any help -- physical or monetary -- from the family members you're trying to get away from. This might make you feel like you owe them something.
Limit contact after the move. You can refuse visitors, stop making phone calls and change your Facebook settings to "unfriend" or "hide" those family members. You might still want to let the kids develop some relationship with your family, but you shouldn't feel obliged to, particularly if you feel the person might be a danger to your kids.
Develop a tough skin. Cutting people out of your life can force others to take sides in the situation and there may be more people talking behind your back. Let those comments just roll off you.