I just recently became a nyab like three months ago and I love my in-laws very much. My mother-in-law and father-in-law are extremely different from my parents but I'm getting through and I'm not talking smack (in my opinion). I mean, people ask me constantly "So how does it feel? Do you like your new family?" or "Is it awkward? Are they mean to you?" But I just tell them what I honestly think, which, in my opinion is nothing bad. I reply and say it's different, I mean I come from a family that talks and yells and we're not very affectionate. But my in-laws only like for people to talk nice, no bad words, soft voices, everyone's nice and caring. My father-in-law is a very picky eater and he has a short temper but I don't intend to make it sound like I'm smack-talking about it. Maybe she doesn't know she's like that, or maybe your sister-in-law doesn't really love your nephew enough to know how to care for his family too. She was more than likely not even ready to be a daughter-in-law in my opinion because when you get married into a family (especially as a Hmong girl), you have to know that you are to love your husband's family as your own and they have to become first priority over your biological parents. I'm still adjusting to that part even now; how do you expect a daughter (more soft-hearted than guys, normally) to grow up with her family and love them to the deepest of their hearts and when they get married, tell them that they have to love someone else's family more? Another thing is, your nephew probably never really brought her around so she could bond with you guys before they married. My husband brought me to his family events from time to time (not like I went every time to be like "Oh~~, I'm going to be your daughter-in-law; I'm so great with such a thick-face I'm prancing around"); it helped a little. I'm only nineteen, and I got married a month AFTER I turned nineteen (it's not much of a gap from your nyab but I'm way different). She wasn't ready to be a daughter-in-law yet and obviously she hasn't grown up from the girly high-school stage where everybody calls each other a b/tch and everybody talks smack about everybody. The best thing to do, rather than ruin your nephew's life by starting back at her, is to just ignore it and move on with your life. If you're not doing anything bad like she's going around saying, then just keep doing what you're doing and don't let the things she says get to you. If someone's like "Oh, I heard your nyab say that you guys are like this and this to her and like that and that; that's so mean of you guys." People are going to believe what they want, you throw your side of the story out there, your nyab throws her story out there, then people just pick and choose. Just let it go.
I'm not saying that your feelings or opinions don't matter and you should just forget about it, but just don't let it get to you. It's the best way to avoid trouble.