Believe me I have stopped talking to people like. I've even stop using my computer completely because I'm scared. But my sister used my Mac just a few minutes and now I've been getting emails from some random person. I don't know what to do.
It's easy for someone to tell you to be brave, it's not always easy to take the advice.
The problem with the person in the chat room is he/she's a garden-variety bully, and he already knows he has you scared. You can ignore him. Maybe he goes away, maybe he makes even scarier threats. But in all likelihood, whatever threats he made or may yet make are
empty threats.
Hacking something like a MacBook can be really, really hard. It's not the kind of thing done to a random person on the Internet - it's done because the target has something of real value - confidential business plans, substantial wealth, potential evidence of criminal activity, etc.
Making threats and trying to scare people, however, is very, very easy. Often, it's plain old social bullying - the bully's only goal is personal satisfaction (bolster his own self-esteem, social position, etc.). Other times, it may be an attempt to fool the target into doing something (like paying hundreds of dollars to remove non-existent computer viruses, or divulging credit card information).
Computers had the Internet have given people with bad intent all sorts of "new" ways to scare and scam people. But the thing is, they're not new methods at all - some have been around in predator/prey interactions for hundreds of millions of years, or more. They're the same techniques that have always been used, adapted for new circumstances. They're based on lies, and as always, successful lies depend upon the victim's ignorance, gullibility, fearfulness, etc. Most people don't know how computers or the Internet work, so it's easy to make up lies and prey upon that ignorance.
Another thing to keep in mind is that people try to connect unrelated things to find patterns, conspiracies, etc. Coincidences happen all the time, and most coincidences are pure, unrelated
chance.
People get email from "random persons" every day of the week. It's junk mail. It may look like it was written personally to you (or your sister), but it's not. Before you encountered this bully's threats, you may have ignored it as meaningless junk. Now that you've been threatened in that chat room, you think that maybe that junk mail is related to the threats.
There's a huge probability that there is absolutely no connection. Your heightened awareness simply means that you're noticing something that has been there all along.