I'm curious about that too! It's very much extrapolating on stuff that's there in the book/show -- nothing that necessarily feels like new information except the confirmation that the relationship between the Nie bros was at one point extremely supportive and affectionate before all the shit went down and Mingjue's personality got warped by the triple attack of personal betrayal/general burdens of saber ghost sect leadership/specific burdens of qi deviation exacerbated by evil oboe. Which is something that's very ambiguous in both primary sources, I think - the book mostly tells us about Mingjue's disapproval, iirc, and the show genuinely gives us very little about their dynamic at all - but certainly plausible based on subsequent events, and also honestly a very MXTX sort of sibling relationship to write ...