I would tell them how you are feeling.
How old is your daughter? I'm guessing she is at least 6. She may have a disability, but that doesn't mean she doesn't know whats going on.
I'd just try to explain that it is hurting you & your daughter to hear them say some of those things. Surely they could consider the feelings of a child, related or not.
My brother has a daughter who isn't his biologically, but as I said, she is his daughter. My parents weren't too happy about it, & are still a bit wierd about it, but when she's around they are nanny & poppy & they love her just as much as they love my children.
I think being raised with their parents makes it hard to deal with for them too as she is Mauri & we are white. 40 years ago, them & Aboriginals weren't as accepted as they are now, my parents were raised that way.
But it has only been 18 months for us. 6 years is pushing it in my opinion.