Regardless of her other behavior, she has no right to comment on your family situation in the manner she did.  I wouldn't have shown half the restraint you did.  I had my first child at 17 (my senior year) and my parents later adopted him.  I've run into the opposite situation when folks see his picture on my desk and I say he's my brother.  But never once have I been insulted in that manner when I explain that he is actually my son, etc., etc.  He knows the whole story and that I love him and that is all that matters.  Being a teenage mother is not the end of the world, as you well know.  I know many women who "started young" and the only difference between us and those we grew up with is that we "got caught" and they didn't.  At least we took responsiblity for our circumstances.  If anything, I think it makes me a better, more understanding mother with my daughters.  It gives me something to use as an example of why they shouldn't give in to certain behaviors - they see the consquences of those choices.  

Kill this woman with kindness and remember that you are the better person.  I know it will be hard to tune her out, but document what adversely affects you and your employer.  Eventually something will be done.

Michelle