EVERY girl you meet is a potential love interest, the game is a harem so you can get ALL girls at once. The only girl you have to get 100% is Sandra, after that you can decide if you want Lucy, Alice, Maddie, Becca, Mary and Elaine as well. All the household girls are Love Interests ;).
And Sandra knows about ALL girls you meet. Sun even meets her later in the game and they talk to each other. There is no secrecy between MC and any girls that he sees - Lucy even knows he keeps dating Tanja ( if you go that route ).