Snobby In Laws Taught their child to hate me.?

my in laws where a Very wealthy blue blooded Type. they didn't care for me at all because even though my family is Upper Middle cl they've always felt that their son shouldn't have married me. In 2007 they both ped away In a Tragic Auto Accident my mother in law was 39 and my father in law was 89. My husband and i Got guardianship of their Daughter soon after who was 7 at the time she's now 12 and suffers from Epilepsy,Severe Asthma and mild mental retardation. From the beginning all we've ever done was love this child like she is our own. She constantly reminds her brother and I that we are NOT her mom and dad , she even says horrible things to our colleagues w at social gatherings. Her parents had always hated me and they taught their child to hate me as well. Before my mother in law ped she said " you bimbo You will Never be family!". the irony is i had to give her eulogy. My sister in law goes out of her way to do mean things to my husband and I. we've never done anything to make this child hate us as much as she does. We go out of our way to make her happy and she does Horrible things she spits at us, yells profanities and slaps Us. We've tried family counseling and therapy and nothing helps. My husband and i are at our wits end with this child. the other family members won't have anything to do with her. They shun her because they feel a child with mild retardation is family embarrment ( to me that's Horrible)