Birth grandparents' inappropriate comments

We've been very open about our daughter's adoption. Our relationship with birthmom is fine; although now that she is married to a nice guy and has two other children of her own, and lives cross country, we don't hear from her much anymore. However, birth grandparents live in the same town with us, and it's with them that I am upset. During frequent outings with the grandparents, they've sown seeds of dissatisfaction in my little girl (5 years old), talking about her "real" siblings (she has three siblings with us) and her "real" mother. They've got her believing that first mom's present husband is her father, and that we took her from her birth mom. We've gently corrected her, but she keeps returning to that story line. I'm stunned that they would do this, but I should have seen it coming; they are very self-centered people. Circumstances (start of school, conflicting schedules) keep our daughter away from these people now, at least until summer. However, the damage has been done. How do I deal with my daughter's new perspectives? I feel so betrayed, because we have always honored her first mom, she has been good to us, but these meddling grandparents have done damage. Just looking for some feedback from anyone who's dealt with this. Thank you so much.