This is a common method to disguise incest in those type of games / vn. In most cases player can change names or roles of characters to family members, if one is into liking those relationships. However, its still annoying to read silly names like "little roommate", "landlady", "household", etc., when u know exactly what is happening there. :D
Just use nicknames . Like "Snowflake", "Moonpie", "Munchkin" or "Buttercup".... that seems to work pretty well (I have also used "Mentor" for the parent role) OR the characters ACTUAL name. Like if the characters name is "Lisa" than put Lisa as their relationship title as well. (other options: Senpai, Stud, Protege... be creative).