My Nephew is becoming a brat is there any way to fix it?

Basically because my brother is tired when he gets home he doesn't have a lot of energy so he pretty much just caves into whatever my nephew wants and he knows that, he'll throw a tantrum if he tries to put him to bed and when my brother is tired of fighting with him to make him go to bed he just lets him...

Luna, you're an adult -- stop worrying about this boys feelings toward you if you discipline him. Take action and show him that his actions have consequences. You might even go as far as to ask him why he's like this, what's happening in his mind. You can't nanny him when you're not there, I get it. But you need to step up, and even get his father involved as well. He may need more than a few toys in the house to play with, give him options. A movie, some Lego, some dress ups. Create a play station (a play area) for him. Don't let this boy walk all over you. As a Pre-K teacher, I'm telling you to step the hell up and be the adult. Hell, when i was little, if I played up like this my aunt, uncle, grandparents, friends parents and police officers had the all clear from our parents to bust me upside my head.