Hi SweetWildCat!
I am really sorry about the situation you are in.

It is tough having to deal with classes and stuff, finding a sorority that you love, and then finding out that it isn't "good enough" for your mom.
A bunch of my friends from HS are in many different sororities at NU, and while I have no idea which ones are considered "crappy houses," I do know that each girl is smart and beautiful. Maybe the reputations that your mother remembers are really different today then they were during the time she went to Northwestern???
I agree with the rest of the girls who have posted. I would find the GLO I best fit in with, and think about what is right for you. After that, I would try and convince my mother that ABC is the right choice for you and really brag about your house. Like, how many girls were involved with your Dance Marathon, how many are on SGA, how many are involved with the Athletic Board, etc. GPA, diversity, etc. Bottom line is that you are you, and IMHO, it is really unfair for her to make this choice for you.
Best of luck! Please let us know what happens.