The LW isn't even trying to be subtle about it. What's even weirder is that LW doesn't dispute that niece has (or at least had) reason to be angry. They don't say it was over nothing. They say the family has apologized, without suggesting it was just to smooth things over. It is abundantly clear in the letter that the family (and not just one individual, either) wronged the niece in some way. The absence of that detail, combined with the niece not wanting LW to know where she lives, makes me think it was something very, very serious.