Guardianship to Adoption...

My husband and I took my cousins two young children into our home in May of 2001 and received legal guardianship of them. Since then, an adoption has been completed for the youngest--our son. The last time the biological parents have contacted us for the oldest--our "daughter"--was June 18, 2002. Now the biological mom is seeking out visitations with hopes of revoking the guardianship and taking back her daughter. My husband and I know that she has lied to her lawyer about things and are very upset by this sudden action. Is there anything that we can do? Our lawyer has advised us to either fight for the biological mothers rights to be revoked, or basically do nothing, and let our daughter go back to her biological mother. It has not been a year yet since there has been no contact, and although child support was never set up through the courts, no support has ever been given. Please help!