Since you don't have personal knowledge/extensive reliable reports of their behavior, I don't think I'd mention anything to DS. I know when I was a kid and my mom over-prepped me for things I just got more anxious.
I would have a lot of different strategies prepared to deploy myself, though. In a way, in-your-face racism (racial slurs, racist rants, etc.) would actually be easier to deal with--forget DS, *I* wouldn't want to sit there and listen to that garbage! I wouldn't give it too long before I said, "Sorry, we're going back to the hotel now," after which I would severely limit contact with them. At some point I would probably have to tell SIL/Bro, "That kind of talk made me really uncomfortable, and I didn't want to be around it, and I didn't want DS to be around it."
In my experience, though, pervasive prejudice can often be more subtle. You can change the subject, you can fail to respond to comments, you can say something like "Oh. Well, that's not been my experience, no," or "Actually, I do like that kind of food." And then you can talk to DS about those comments later.
I tend to be pretty non-confrontational. Saying, "That's racist" is pretty inflammatory, especially in someone's own home, when you don't know them very well. To me it's more the opening of a debate, not a way to shut down a conversation. I'd rather just "avoid the crazy," personally. Others are able to handle it more assertively, while still being polite.