A woman in the situation you describe would be technically free to pursue a consecrated vocation. However, she would have to consider what is in the best interest of her children. Even if she doesnt have custody, there is the reality of her motherhood and I (this is just my personal opinion) think that generally speaking she should be available for them, especially in their teenage years, for it is very hard to foresee their needs and reaction when they come to that age. Now, there are several factors that affect this judgment, such as the fathers situation, the stability of the new home, etc She would not want her availability to in anyway jeopardize a situation that may be settling down.  There is also the opinion of the order she wants to join, and if they think it is prudent. Sometimes an order will allow a person to be associated without being an actual member while time goes by and takes care of other circumstances.

God bless.