How to handle my mistake and my mother in-laws racial bias?

you failed to mention ages. but i will say that her parents grew up in diffrent times and although are trying to show an open mind it is diffilcult for them. not excusing their behavior. I grew up hearing the n word at thanksgiving dinner because back then there it was common for white people to use it and even though they used it i was raised to accept everyone but always knew dating a black woman would cross an unsppoken line. but to your problem although you will never forget what they have done they still are your wifes family and obviously accept your child so let go f your anger and forgive them maybe over time even at thier age they will learn to accept you as a person not as the black man who stole thier daughter. give them the chance. turn the other cheeck god bless