What do you mean it's never brought up? It's mentioned every time they use "family" or "brother" or "sister" or "mom" or "dad" in various interactions, which as you noted yourself, the game does use. E.g. MC can literally say "she is my sister" about Bella when inquiring at the tattoo parlor.
And yes, if you defined them as acquaintances then he'll say "she is my acquaintance" instead, but it doesn't change the fact that if you define them as family then it does get brought up every now and then and they are referred to as family. It's just like i said, people being family members doesn't change basic interactions between them in some dramatic ways, compared to interacting with friends or whatever.