Use of gendered language is marginalizing to nonbinary people.
It's easy to figure out why these changes are being made in particular, once you take a position of empathy.
References to brothers or sisters, or mothers or fathers, is offensive to nonbinary people? In all seriousness, not trying to start a flame war, why is that? Is a reference to hair offensive to a bald person?
Using the perspective from the OP, it's a kids' book, using a couple of different synonyms might help them better understand what they're reading and not walk away believing that there's something wrong with their family or themselves.