You're step-sister Grace has gone through... Well, "a stage in life" as your mother calls it

She's changed from her normal personality into a more complex and depressed one, she also seems a lot more.. lustful as you've seemed to notice

You hear a knock on your door before Grace just walks in without asking

Grace: "Hey little bro, I'm bored, wanna head out and grab some lunch?" she says playfully, but you can tell she isn't going to take a "no" for an answer