How do I handle people’s negativity about our daughter’s open adoption?

I don’t know if anyone else has a similar situation…but, until we adopted our little girl 4 years ago…no one in my family or my husband’s family had ever adopted a baby.  So it was new, unfamiliar territory for everyone.  And when they found out we were entering into an OPEN adoption, everyone pretty much freaked out and fought it.  Especially my parents.  My husband’s parents have since “come around” and have not really said too much regarding our daughters birthfamilies. However…my parents (esp. my mother) continues to say negative things about them, calls them “THOSE people”, and really tries to persuade us to cut all ties with them….or hopes one day that they will move on with their lives and never want to see her again.  I have been patient, I have tried being firm, I have tried “laying down the law” about how the promise my husband and I made with the birthparents is a lifetime commitment that we intend to keep.  My mother especially insists that doing so will be detrimental to our daughter in the long run…(she is basing this opinion on the fact that we’ve had many challenges with the birthfather and his parents.)  I know she doesn’t mean to do harm or really mean to hurt me, and I know that she is coming from the place of being a mom and not wanting her children to get hurt. But I really want her to get passed the “old school” view of adoption and see that what we are doing and why we are doing it is really a good thing and really is healthy and positive for our daughter. Nothing I have said has sunk in or persuaded her in any way.  Does anyone have any suggestions?  Any comments would be greatly appreciated.