Hi there and
to the forums,
I'm sorry that this happened to your budgie.
However, you need to be sure that your brother understands that animals are innocent creatures and never deserve to be punished in lieu of their owners. Additionally, budgies are very fragile birds that can be killed even by "swatting" at them. You need to get your parents to explain this to him if he doesn't listen to you. You are very fortunate that his tail feathers are the only thing that were damaged. Budgies can eject their tail feathers when they feel intense stress; a fast-moving sweater sleeve mimics the movements of a predator and caused him to eject his tail feathers.
He may be off balance for a few days as he adjusts. He will be very stressed right now. Be sure to monitor him closely to ensure that he was not harmed in any other way. If he does seem unnaturally dazed, lethargic, etc. then you need to take him to an avian vet right away.
You need to consider housing your budgie in a safer environment where your brother doesn't have access to your bird.
Meanwhile, be sure to read through the forums' many budgie articles and stickies, as they include all the information needed for you to stay updated on the very best of budgie care practices.
If you have any questions after reading through everything, please be sure to ask as we'd love to help.
Please keep us posted on how your budgie is doing! We hope to meet him soon