For your mother’s sake, look for a community that has good support for her. This does not necessarily mean that there should be a large number of immigrants from her own country or language group, but rather that she has good access to English language instruction, public transportation (if she doesn’t have a car), and activities that interest her. Otherwise she will be overly dependent on you and your younger sister for everyday life. This will create imbalances in the family relationships, and will make it harder on your younger sister when it is time to go away (if she does go away) for college.
Not to be too nosy here, but how did she get the Green Card? If it is through a family member, then would it make best sense to live somewhat close to that relative? That might make her adjustment easier.