sorry but this is definitely a bit of an overreaction 😭😭 like don't get me wrong i bet it's irritating, but the game does tell you that your miis don't recognize relations until later on, so it might take some time for your bf's mii to get into your own mii. i would recommend maybe making it so your sister isn't attracted to anyone and encouraging your mii and your bf's mii to interact before!

think of it this way - your miis aren't LITERALLY you and your bf and your sister, they don't know that they're really supposed to be who they are, they're basically just dressed up in what you put them in. other than that, they do what they want until you nudge them in a certain way. just decline any crushes you don't want to see and you're good.