A whole lot of the action in Luna happens at the homestead, which isn't quite as safe as you'd think. Luna has to go by Liam all the time when she's around her parents, and Regan also has to keep up the charade. On the surface they're a nice, normal American family, but things aren't actually going so great in reality: Luna is dying from her secret, their parents don't get along, and their mom hardly seems involved in the family life at all. Despite it all, Regan and Luna keep their bond because they truly know the power of unconditional sibling love.
Questions About Family
Why can't Luna tell her mother and father that she identifies as female?
Does Regan end up taking the role of younger or older sister? Why?
Why doesn't Luna's mom tell her father that she doesn't want to be a boy?
What kinds of rules are there in Luna and Regan's family? Why can't they be challenged?
Chew on This
Because Luna and Regan's father is a conformist and is very conservative when it comes to gender norms, he willfully ignores the evidence right in front of him that Liam is not your average boy.
In order for Luna to completely live as her own person, she has to separate from her family structure and go against what is expected of her.