1. Regional Accreditation is what I look for.  If a school is accredited by one of the Associations listed here, I look no farther with respect to whether or not the school is a "diploma mill."   If an undergraduate degrees require specialized accreditation, such as Engineering, I look for ABET accreditation:

http://www.elearners.com/resources/agencies.asp

The easiest way to get the answer to the general accreditation question is by searching the Department of Education's new website at:

http://ope.ed.gov/accreditation/

If this is the college, it looks ok:

2. The next question your sister might ask herself is, whether or not this the best online college for her educational goals?  Is it important for her to go to a school with a "name." Is she hoping to go to graduate school?  Will this school hurt her chances for getting into graduate school or getting a job?  How about the price - is money a concern? Is the school for-profit (I am still somewhat skeptical of these schools unless there is no other alternative).  For-profits are in the "business" of education.  That can be good news as well as bad news.  In many cases the for-profits are ahead of traditionally delivered online programs with respect to delivering student services at a distance (admission, registration, payment, bookstore, library, academic support, advising etc).  If they don't have excellent services, they will lose their customers. 

In addition to the "name," the real question is whether or not the educational experience is comparable to online programs delivered through traditional colleges.  If this is a concern, I would look for ways to take one online course as a "student at large" before making a commitment to the school that doesn't have a recognizable name.  Remember, if your sister gets an online degree from the University of Illinois, for example, it is appears as any other U of I degree. 

I have a tendency to explore traditional colleges that offer fully online programs before selecting a school that is fully online or where most people consider it an online school, even if it offers some on campus courses such as University of Phoenix.  I believe there is still an elitist attitude with respect to non traditional schools (Capella, Walden, and many others).  Depending on your sister's situation, that may or may not be an issue.  When working with a student who is looking for a Bachelor's degree for advancement at their current company, this is less of an issue.  For all other students, I suggest they call potential employers or graduate schools to assess the possible consequences of having a degree from a non-traditional school on their resume.  Just like all other decisions that have to do with the "name" of a school, if your sister interviews for a job or graduate school admission with a person who has a degree from a for-profit/non-traditional college, this becomes an advantage. 

3. Here are some additional resources I use to find online programs and help students decide if these programs are good for them.  I am not recommending any programs included on this list: