There’s an old saying: “Fool me once, shame on you. Fool me twice, shame on me.” The point is that your family has taught him that he can do as he pleases with no negative consequences for inappropriate behavior. You didn’t give me enough information for me to make a guess why this got going. People can enable others out of fear, out of pity, because they feel they owe someone something, because they don’t know what else to do. Sometimes people enable someone in the hopes that if he gets enough support, he will straighten out. It rarely works but you can’t fault good hearted people from trying.
Sometimes parents and older siblings at first enable a younger sibling because he’s cute or because they feel sorry he can’t keep up. What starts with the best of the intentions becomes a nightmare as the kid gets older and still expects everyone to see him as the Little Prince he once was.
Perhaps you have an idea why your brother’s behavior is tolerated. If whatever started it still exists, chances are your parents won’t change their acceptance of his rude and entitled behavior. But that ‘s your parents’ decision.
You are 26 — an adult. You probably can’t change your family’s way of operating since (judging from your father’s comment) they don’t want to change — even though they complain about him. You can change your response. Better yet, you can work, save your money, and set a goal for leaving them all to sort it out together like your sister did. You don’t have to be a part of it.
Meanwhile, stop cleaning his room (even if you are paid). Stop putting away his laundry. Put shades on your french doors so light won’t bother you. Don’t respond when he comes to you for something he wants. Let him know that he’s an adult and you will treat him like one even if no one else does. Then do it.
If your parents would like to change your brother’s treatment of the family, I suggest you all go together to see a family therapist. The therapist will make it clear that you all can’t change him. But you can change how you all respond to him. The therapist will give you guidance and support for doing so. If you do, and do it with consistency over time, your brother will no longer get what he wants by being obnoxious. He will have to either change or leave. He’ll hate having to finally grow up, but it could be the best thing that ever happened to him.
I wish you well.