It seems downright archaic that a sorority would have bylaws against an active with children. Is the problem the conflicting commitments of sorority life and children, or is the fact that children are an unmistakable sign of sexual activity? If the latter's the case, how can they make sure that every active is a virgin?
I feel the MOST IMPORTANT issue is whether or not she's been lying, and what she's lying about. Otherwise, please come up with a "better" reason to depledge her, if you have to at all.