I disagree with Valkyrie and Beofett. I am a programmer but I abhor reading articles written in l33t. Even personal ones. If you dig up the history of l33t you'll learn that it's a degradation of another form of writing in the earlier days of computers when 8-bit ASCII glyphs were used as a decorative set of characters as an alternative to the normal Latin alphabet.
This was alright because you could still read sentences clearly. For example a simple interrogative like
How are you doing today? might be written as
Hôw ärè ÿøü ðõîñg †°ðåý?. It's legible (albeit a little difficult to read). However l33t is extremely illegible as there's no one-to-one mapping between characters (e.g.
H\/\/ r yu duin 2dae?).
I'm going off topic. The point is that a proofreader is not helpful in this case. It'll correct your mistakes for you, but it won't help you learn proper grammar. You learn by making mistakes. If you give your sister a proofreader then considering she does use it (which she may simple refuse to do --- it's not "cool" in her own words), at best it'll only make her dependent on the proofreaded to write her own essays.
You don't want to make her sit down and try to explain why it's better to use proper grammar. That is lecturing, and lecturing is only effective when the audience is willing to learn (like in a Google Talk or an open seminar). It rarely ever works in classrooms and in adult--teen discussions.
What you want is to make a statement, and you should be very clear in the message you're trying to send across. You want to be as brief as possible, but you still want to make an impression. You don't want to lecture her on why not to do something. Instead you want to get a point across (in this case -
don't use l33t - it's not cool). The why is for her to figure out on her own.
There are several ways to do this. Beofett got really close to finding this. He suggested that you simply ignore her when she sends you a message like
if u c ne $ in d car plz brng it 2 me. That's a start, but I assure you it will not work.
If you ignore her then it'll only make her annoyed. Consider the following conversation:
Your sister: Did you find some money in the car?
Your sister: Thank you. Where is it?
You: In the car.
Your sister: But I told you to bring it to me.
You: I know.
Your sister: So why didn't you!
You: Because I didn't like the text message you sent me.
See what you're doing here? You may think you're sending the message
I didn't like the text message you sent me across to her, but she doesn't get that message. What she receives is the message
My brother doesn't care about me.
Then she'll get annoyed and it'll turn into an argument. She might ask
Why couldn't you just bring me the money?, and you might retort
Why can't you just type proper English?; and instead of magically solving problems it'll just frustrate both you and her.
Ignoring a person is the worst way to send a message across. Especially if the person you're intending to send it to is a woman. Women only ignore others when they're cross with them, not when they're trying to be supportive or trying to help them. Don't ignore her messages.
Instead when she asks say something like this:
Your sister: Did you find some money in the car?
You: Do you mean the $20 on the back seat?
Your sister: (happily) Yes, that.
You: Oh, I'm so sorry - I used it to buy myself a McDonald's burger.
Your sister: What? But I sent you a message asking you to bring it to me.
You: You did? I didn't really understand what you meant to say.
Your sister: Oh.
You: Hey, I'm sorry. Here, you may take $20 from my wallet alright.
See, it's not that difficult. Here the message you're sending is pretty clear -
don't use l33t to talk to me - I don't understand it. You didn't ignore her. You didn't fight. You got an excuse to eat a McDonald's burger, and she got her money.
That's still however just a part of the message you're trying to send to her. Do this regularly and she'll talk to you in proper English, but she'll still keep using l33t on Facebook and with her friends. The next step is to demonstrate to her that speaking in l33t is not cool.
What's the best way to do that? Interact with everyone around her in l33t. Initially this may seem counter productive but it'll work. Talk to her friends, your parents, her friends' parents, her teachers, etc. in l33t. Make it a habit. Eventually it'll pique her interest and she'll ask
Why are you talking like that?. Then you may respond by saying
Because it's cool.
In her mind she'll decide for herself that it's not cool to talk like that and eventually she'll grow out of it herself. It's important to let her grow on her own. Yes you can nurture her and you can make her aware of your opinions, but you can't force her to accept your point of view and expect her to change her behavior overnight.
Hope this helps. =)