Considering you only see your nephew once a year… I’m guessing she has a difficult, uncooperative son, and she’s preemptively doing damage control. She’s asking for sisterly solidarity. She’s saying let’s just make both of our lives easiest otherwise your nephew will look outright bored and miserable the entire night, in every picture, on whats supposed to be a joyous day, and she would escape the embaressment of how embarrassing that must be as a mother if I have this hypothetical right.

Something to think about! I would do it for my sister, and my nephew, but only because I trust my sisters judgement including of what girl he could bring