The problem I've found with people like that is that, essentially, they don't think you're human; yelling and showing verbal distress is just funny to them and encourages them to be even nastier to you because they like getting you worked up. And I've had a lot of those people in my life. All you can do is either hurt them physically (with may or may not expose you to, well, what's happening to LW; see also, for example, reactive abuse, which is how I would frankly classify LW's fiance's response; even if this is the first time that particular abuser inflicted that on her, she's been getting enough of it from a variety of sources for long enough that she and LW specifically warned the family not to do it!) or find someone they do think is a real person whose feelings actually matter to them, usually someone with some kind of power over them, to impose some consequence on them that they do find meaningful. Shouting and showing upset is the response they want.
Now, perhaps either LW's fiance or LW coulda-shoulda-woulda turned to niece's parents or LW's parents (or if the family has a "head", however informal, someone who's understood to lay down the law about family dynamics) and frigidly and contemptuously (not upset/angry, because again that is what LW's niece wants and she will enjoy their distress) said something along the lines of, "Are you going to let this nasty little brat act like that to a prospective family member? Because that's a terrible impression you're giving of this family." Though given the way niece's parents reacted to the whole situation, and for that matter the fact that LW's niece thought it was perfectly fine to do a specific hurtful thing that she had been told not to do says that the parents at least are perfectly fine with the behavior, though other adults in the family might not be.
But the thing is that LW and fiance thought that they had headed this problem off at the pass by asking LW's family not to bring it up, so the level of blindsiding there probably would probably see fiance's action clear the standard for irresistible impulse in at least some jurisdictions--- she was expecting to be safe from that and then here's this person inflicting it on her.
no subject
Now, perhaps either LW's fiance or LW coulda-shoulda-woulda turned to niece's parents or LW's parents (or if the family has a "head", however informal, someone who's understood to lay down the law about family dynamics) and frigidly and contemptuously (not upset/angry, because again that is what LW's niece wants and she will enjoy their distress) said something along the lines of, "Are you going to let this nasty little brat act like that to a prospective family member? Because that's a terrible impression you're giving of this family." Though given the way niece's parents reacted to the whole situation, and for that matter the fact that LW's niece thought it was perfectly fine to do a specific hurtful thing that she had been told not to do says that the parents at least are perfectly fine with the behavior, though other adults in the family might not be.
But the thing is that LW and fiance thought that they had headed this problem off at the pass by asking LW's family not to bring it up, so the level of blindsiding there probably would probably see fiance's action clear the standard for irresistible impulse in at least some jurisdictions--- she was expecting to be safe from that and then here's this person inflicting it on her.