Clean your act up.

Dear cousin.

Although I've never seen you rage. I know about it. AND OH. IT'S BAD.
It's not for me to tell you how you should act. But seriously, you're how old? LIKE .FRICKEN 20 in less THAN A FRICKEN YEAR.

She has no confidence to speak up for herself, and she is my family. And so are you. But seriously. Next time reconsider your fricken actions. She's your FRICKEN SISTER. And you still decide to treat her that way. Do you know how much she loves you? Obviously you DON'T. I know that I will regret this once I say this to you. You are well-off. There are people who are MORE unfortunate than you and I will ever be. And that's a FRICKEN PROMISE.

Before you say "Oh, but we have enough cash" etc etc. FRICKEN HELL. PRESERVE YOUR MONEY BITCH. AT LEAST TRY HELP YOUR PARENTS SAVE UP FOR THEIR FRICKEN RETIREMENT. FAH. Do you need any more common sense instilled into your body and mind? Coz if you're parents aren't doing a well enough job of that, I'll be helping them. HAPPILY.
Yeah, wallow in self pity when we tell you off for it. But seriously, sharing an apartment with your sister. Is it REALLY that much of an annoyance to you?  She's going to be your family WHETHER YOU LIKE IT OR NOT. So I GUESS that YOU'RE JUST GOING TO HAVE TO LIVE WITH IT. Do you know how much I've wished for an actual blood related sister? If you don't want her. I'll take her. GLADLY.

Gosh I hope my standing up for your sister hasn't made you want to start raging at her and me. Coz you know that we see each other each week, whether you like it or not. So I guess it's actually good to have such a tight-knit family in a way. Run away. Oh where would you go? :)
NO WHERE.
Once your parents get their hands on you. They'll probably be metaphorically strangling you.

Hope you clean up your act soon.

Love,
Your cousin.