I'm VERY late to this discussion, but I fully agree. Ideally, they would have baked a third gender option into the game at its foundation, but they didn't and as much as I wish they would now, I am pretty confident that they never will. That said, I do still think they might come around to making the gendered relationship names an option in the customization menu (which would be awesome).
As far as terminology goes, parent, child, and sibling are all great, but there is already a neutral term for aunt/uncle and it is ommer. And while I know a lot of people love the term neiphling, as an enby myself, it isn't my preferred term (neiph would be ideal for me).
Just to add, I just noticed your section about Growing Together and would like to add some of my options (as a non-binary person myself):
