Grandparent's treatment of my children?

I am a half Japanese ( on my father's side) and part American and part Hispanic woman married to a white man. I am average height and have somewhat dark skin, blue eyes, and dark, curly hair. My husband is somewhat pale with blond hair and yellow gold eyes. We have two beautiful twin daughters, one of which has lighter skin, blonde hair, and blue eyes, and the other who has my husband's eyes, darker skin, and my hair. They are both babies still and we love them both very much. My husband I have noticed recently that both my husband's mother and my own, fawn over the less ethnic looking child of our fraternal twin daughters. They say things like "Oh, she's so pretty," and "Her skin/eyes/hair is lovely." They also ignore my other daughter. Both my husband and have spoken to each of them and they have not listened at all. This disturbs both my husband and I deeply and we're considering revoking the right of both of them to see my children if this behavior continues. Is this just? I don't want to deprive them of visiting rights but, currently, I see no other options. Suggestions and alternatives are appreciated.