I have a problem. A few years ago, my mom died. About a year after that, my dad started dating the mom of one of his baseball players, a guy I'm going to call Mark. 6 months later, my dad and Mark's mom got married.
Now, I love my dad, and I want him to be happy. But I hate living with my new stepfamily. Why? Because Mark is evil. He's one of those people who has everyone under his thumb, his mom, his teachers, even my dad. But under the surface, he is just cruel. He is constantly messing with me. He goes through my things, invades my privacy, spreads vicious rumors about me and still expects me to love him. Whenever he wants something from me (like to lend him 20 dollars) and I refuse, he'll go into my bedroom and take something of mine (like my modelling portfolio) and refuse to give it back till I give him what he wants.
About 2 weeks ago, Mark lent his car to a friend, and they basically wrecked it. Later that week, he demanded that I lend him mine. I of course, refused. So he snuck into my room in the middle of the night and stole my mom's locket, the one she gave me before she died, from around my neck. He told me that if I didn't lend him my car he'd flush it down the toliet, and I knew he wasn't bluffing. He's done it before (dropped my phone in water, shredded my pom poms into confetti) so I handed over my keys. Please tell me how I can avoid being manipulated by him again.
Geez, Sparkler. Did you write this letter from inside a bad teen movie? Because if so, I think I've seen this one before... in which case it's going to turn out that your stepbrother is secretly in love with you. And also that you have an extra set of teeth in a very unfortunate place.
But if this is real, then it is horrible. And there are two important steps you need to take right away, before you do anything else, in order to retake control.
First: TELL. YOUR. FATHER.
Tell your dad that Mark has been invading your privacy, taking your things, and using them to extort money and favors from you. Because as much as I feel for you, I also can't understand for the life of me why you haven't done this already. Unless there's something really peculiar going on—like, say, you have a history of baseless accusations and delusional behavior—then your dad has every reason to take you seriously. (And if he doesn't? Go to him with evidence; even a skeptical parent will have a hard time ignoring an audio recording of Mark's next outburst of douchery, or finding one of your treasured belongings in his room/pockets/car.)
This isn't just something you're doing for yourself; your family members need to be aware of this for their own sake. At best, Mark is dealing with some deep-seated issues (possibly stemming from resentment over your families having blended) and needs serious help; at worst, he's a freaking sociopath. Either way, it's important that your dad and his mom know what's going on with their kid.
And second: LOCK. YOUR. DOOR.
If you're old and independent enough to have your own car, then you're old and independent enough to go to a hardware store, buy a doorknob with a keyed lock, and install it on your bedroom door. (You may also want a basic, doorjamb-mounted slide lock you can engage when you go to bed at night.) Or, at the very least, buy a large, heavy, secure trunk and a sturdy combination lock that you can use to safely stow anything you care about.
I know this seems like a cold and pragmatic solution to what's clearly a very emotional issue, and one that's deeply unfair to boot. You shouldn't have to do this; your home should be a safe place for you and your things. But it's not, and until something changes, the $30 it'll cost you to secure your belongings and/or your bedroom is a small price to pay for neutralizing Mark's ability to harass you—or at the very least, making it a lot more difficult for him to do it without getting caught. (Even if he somehow managed to steal it, he'd be hard-pressed to explain to your folks how a large, padlocked trunk that's clearly marked as yours and full of your most prized possessions found its way into his hands.)
Those are your first two steps. But there's a third one, too—and it's the answer to your question about how to keep yourself from being manipulated. And that is: Don't allow it.
And the way to do it is to secure or back up the things that are most precious to you, and let go of the rest. Because—and repeat after me, now—they're only things. They can be replaced.
No, really, say it out loud. Say it as many times as it takes to sink in.
And once you've said this to yourself, you can say it to your stepbrother the next time he tries to extort you. Shrug, raise your eyebrows, and say, "Whatever. They're only things." (And, in the same blasé tone: "But I'm sure you'll enjoy explaining to our parents why it is that my belongings keep disappearing, getting damaged, or winding up destroyed.")
Really, it's as simple (and as difficult) as that. Mark can only blackmail you with your own belongings as long as you let him do so; whatever power he has over you is a power you give him, and that means it's a power you can take back.
FINAL NOTE: In most situations, following the advice above should solve your problem. But if the unthinkable happens—if nobody believes you, and if this doesn't stop—call the domestic violence hotline at 1-800-799-SAFE(7233), explain the situation, and ask about your options. What your stepbrother is doing is illegal and abusive, and you don't have to tolerate it.
Have you ever had an evil step-sibling? Share in the comments! And to get advice from Auntie, email her at firstname.lastname@example.org.