Racist in-law

My older brother married outside of his race 7 years ago which was no problem our family welcomed her with open arms and she was the sweetest thing ever but my little sister wasn't convinced. My sister is typically a troublemaker, doesn't get along with anyone she went to visit our brother and I got a phone call at 2 a.m these fools are drunk at least from what I could tell but my sister and brother are arguing because she got into an argument with his wife and she called her the n word with the er at the end. I thought it was outrageous and brushed it off, told my sister to stop exaggerating. A few months later I score a job interview close to my brother he's 2 hours away from me I thought it'd be a good idea to be near family so I stay with him for a few days because it's a two part interview. I get there I am welcomed by both my brother and his wife he's going on and on about my cooking. He can't wait asking me what I'm going to cook I could tell his wife felt some type of way but Lort I did not what gonna unfold over the next few days. I go to the grocery store to get a few things so I can cook for my brother like he asked. I made fried chicken, greens and mac & cheese she didn't eat it. I cleaned up well and the next day before my interview I made breakfast at my brother's request she didn't like that either by now its all over her face she pulled him to the side he comes back frustrated but acting like nothing is wrong. I go to my interview, come back to their house my brother is gone but his wife is still home she's not opening the door I see her car in the garage and I can hear her inside. I'd been calling, texting and knocking forever. So I call my brother, he makes a phone call and tells me to come to the dealership he'll be going to lunch soon so I left. He didn't say anything again and now I'm just like what's going on he says his wife is in one of her moods whatever the hell that is so I ask if my cooking is overstepping even though he asked me to do it he assures me that it's not but I feel like it is so I offer to teach my sister in law how to make spaghetti the way he likes it that night. Let me tell ya it was a disaster she didnt do anything I said just made it her way no onions or basil just meat and tomato sauce. Regular tomato sauce too not homemade or even Ragu she also prepared it differently meat on top of noodles. I fried the fish and made the corn on a cob. Dinner seemed to be going well until my brother and I mixed our spaghetti she couldn't hold it in no more she interrogated us about it. It really frustrated her that we did that and the flood gates opened. I always washed my brother's hair in the sink he has long hair and he's helpless he's never washed his own hair. I don't know why but black people wash their hair in the sink always so she complained about how I washed his head and mine in the sink. She complained about grease taking up space in the bathroom this is just day 4 of my visit so it seemed like she already had issues before I came. No racism popped up in my head even after this because it's an interracial marriage I expect there would be some cultural differences but they still fell in love and got married anyway so I figured they worked it out. Second part to my interview went well I came back to them arguing she's accusing him of cheating and he's not he just wants to go out with his frat for a little while. I sat on the porch and stayed outta married folks business his 8 year old daughter is coming so that's where I put my energy. Hadn't seen my niece in a while so I'm excited her mom pulls up, let her out, I text my brother and go greet my niece. I'm not sure if they're still arguing so I'm distracting my niece and making sure she doesnt hear them. His wife comes stomping out the house and for the first time she rudely asked me why didn't I come get her so she could get Mariah from her mom. I was confused she walks off cursing in front of my niece. I'm standing there puzzled and now uncomfortable I ask my brother wth is his wife's problem, what did he do, etc? He shows me their texts and oh boy... even before I came it was like reading a mark Twain book. N word every other sentence. She also tells him that she's genetically superior to him and I am appalled at first but then I get angry like you're really putting up with this? Along with my niece? He said it was just her moods she didn't mean it and she'll be different. I tried to give her the benefit of the doubt but that chick is evil. I packed my stuff and called my interviewer to tell her I was no longer interested. It's been about two months I've barely spoken to my brother and I'm still digesting it I can't wrap my mind around this. I apologized to my sister but moving forward... how do I do that? First time facing a racist well not really facing her but actually knowing one has my mind all discombobulated.