Tomboy stepsis is a superhero?
Don't act so surprised big bro, I'm always leaving at the most random times... Did you think I was just hooking up all the time?
Kara has been your little sister almost all your life, ever since your father passed away rock climbing, your mother ended up remarrying when you were only 3. You both quickly moved in with them into a 2 story house inside a good neighborhood. Your mother Hailey never told you the details of what happened to your sister's biological mother, you thought to yourself it would be too awkward to ask, so you eventually let it go and grew up as a happy family.
Around when your sister turned 15, you overheard your step-dad Clark explaining to your mom in the kitchen one night from upstairs that, "Kara is special, she might have to leave every now and then to take care of things. It's okay Hailey, I wish I could say more... but trust me she will be fine." You never asked about that either.
Now you're in college together with Kara in a dorm house you both rent, and you caught her sneaking out again. Only she was wearing strange clothes...