Embarred of my mom?

I've never been sure of what's wrong with my mom--growing up, she was really depressed, and I moved out of her home early. She doesn't take good care of herself, she acts very socially awkward, says wrong things at the wrong time or keeps saying the same thing over and over again, and is very reclusive. She doesn't have, or want, a close circle of friends or a spouse/boyfriend. She doesn't try to be close to her brother or anyone in the family. She also has a bit of delusions of grandeur, I believe. Either as another symptom or because of this, she's also just plain mean. I've tried to deal with this as best I can, but now I'm engaged and my mother and fiance's mother are about to meet. I, by all accounts, have "escaped" my mother's world and am not really anything like her (my dad taught me lots of things and is a member of normal society). My fiance's mother is a lovely lady and I can't bear the thought of her feeling like her son is marrying into a crazy family.