Evicting a roommate who refuses to leave. What legal steps do I need to take?

You should find a local landlord tenant attorney and call them and inquire what your options are. I'm not sure if it's really illegal to discriminate against someone for having children as previously stated though. I would guess that if you put the effort forth to find a landlord tenant attorney and then have them send your roommate/tenant a legal letter she'd probably not want the trouble and instead would find somewhere new to live.