Well, your examples are clearly examples of awful behaviour. Without knowing what was said here, we can't comment. As someone who is also not white, I do come across a lot of white people getting offended at inoffensive things, which winds me up, because not only are they inoffensive, they get so strung up on the stuff that isn't stuff, that they let the real racism slide by like it's acceptable.
People need to be speaking up against actual racism, such as the examples you've described. I'm really sorry that couple let you down so badly - sometimes it's not witnessing racism from strangers that's so upsetting, it's when our friends and acquaintances show themselves for who they are, and it's not who we hoped.
Anyway, back to to the OP. Is SIL the daughter of MIL? In which case, if they're both racists, it's hardly surprising?
You say you can't go NC because of SIL's DC, but also that SIL's DC is an adult. If that's the case, why do you need to speak to SIL? Surely as long as the DC is talking to the mother, you don't have to?
And if you can't go NC, can't you go low contact?
I think if you've indulged your MIL though thus far, it's going to look odd at you suddenly coming down against your SIL. You should really be treating all racist views the same, not hand out free passes because of who's saying them.