Yes you can already technically change what you call them, but grammatically and writing wise they are very clearly relatives and a copy-pasting a different title doesn't do anything besides make it feel sloppy. Scrap the "who are they to you" text bubble, and make it a option between "your family" or "your nerdy friends family" and just alter the dialogue between the 2 so they make sense. World wise nothing really has to change, you can be living with your friends family after college for whatever reason and pitching in because they're in a bad spot but still putting a roof above your head. If you don't select them as your family then scrap the lines where they say they've raised you their whole lives or the lines that say "i'm your ____" but only makes sense if the word was supposed to be mom/dad/brother. The earlier changes like these are planned in development the easier it will be to plan around it going forward.
The game is amazing and the art is fantastic so I'm saying this because I really do want this game to be awesome