It is not too late to turn this things around. Having said that, if the person who is his primary caregiver is not on board, then it's going to be hard. It's a lot to ask your sister to take this on. It will require energy and staying power. Does she have it?
I think it can even change if he isn't with her all the time. She just has to get tough and mean it and once she lays down the law she needs to stick to it to the letter. My nephew was a holy freakin' terror. His mom wasn't way to strict on small silly things like his be being made and then not strict enough on the ones that really counted like obeying and telling the truth. His room can look like a bomb went off as long as he doesn't look at me and lie and then turn around and do whatever he pleases regardless of what I say.
When my nephew came to stay with me on the nights his mom worked we made it super simple and there was one rule. I was the boss. Whatever I said was the word for the day. To cross me was peril. So the only rule was: Do exactly what I say right when I say to do it. If he didn't, he hated life for a little while. By the end of two weeks when he walked into my house he was a different kid.
Each kid is different. If he really likes the tv and computer take them away. I mean completely unplug them from the wall. No matter what kind of fit he throws, SHE still has to be the one to hook everything back up, so the punishment lasts as long as he does.
Put it to your sister like this. If she is stern enough now, then she won't have to be later. Usually with kids, if you prove it consistently, eventually you don't have to prove it anymore. They know and they comply.