You just have to show and tell by your own positive example! I remember when I was a little girl we would go visit my grandfather. Now- my grandfather would ABSOLUTELY not have considered himself a racist. He had served in WW2 in the Navy with black men and often spoke fondly of his comrades and what hard workers they were, great guys, etc. BUT- he thought NOTHING of using the 'N' word, making totally racist statements about 'ghetto babies' etc.!!!
Whenever we went to visit, in the car on the way, my mom would give my brother and I a little talk. She would say that grandpa used words that we should NEVER repeat and if we had a question about them we should ask her later on the way home. She said that we love grandpa and we respect him- but that does not mean we agree with everything he says or thinks.
She always made it perfectly clear that loving and respecting him as our grandfather was a totally separate thing from buying into everything he said! I learned that lesson as a child and it still holds true for me in difficult situations (political disagreements within the family, etc).
Maybe you can talk to your stepkids in that way- the way that the other part of their family views the world is just ONE way of looking at it. You don't agree with it.
It doesn't mean you are telling them not to love or respect their mom or other stepparent, but it does mean that you will not tolerate bigotry in your own household. They need to really grow up and look at both ways of thinking and when they are a little older, hopefully they will see the right path in YOUR good example!
You could also help them by looking at their family tree or yours or most peoples- hardly anyone is just all one thing anymore! Most of us are 'mutts' and are proud of ALL of our various heritages! It's a good thing to have so many wonderful cultures and traditions, etc. Try to emphasize all the positives to them of these various backgrounds and hopefully as they get older, they will just appreciate it even more.
I have a son with a great stepdad- good for you for trying to do your part to raise these kids right! Good luck!