What are some ways i can make my son and step son more comfortable growing up with parents in a biracel marriage and having a biracel sibling?

I think you need to feel comfortable about it first. If you're fine with it, and in your family's day to day life you all act OK about being a biracial family, then it will be alright. It starts with the parents.

Yeah both of my caucasian parents married African American's after their divorce, you're making up an issue where there is no issue. Firstly, I highly doubt your kids are going around school advertising that their parents are biracial because it doesn't matter to them. Secondly, most children these days won't care and adults SHOULD have enough self restraint not to say anything if they find it odd. IF and issue arises then you should deal with it but really I'd be more worried about the biracial baby you're about to have and helping it fit in and feel comfortable in it's own skin because a lot of biracial kids have problems identifying with other people because they're neither black nor white.

I am biracial and it was no big thing as long as you do not make a big deal out of it just treat them the same and when they see your comfortable with it they will be as well.