That is one of the most common reasons given, and it is also one of the more fixable ones. So take some heart.
There are two broad routes from here.
Appeal the decision. You are arguing the refusal was wrong on the file as you submitted it. Your refusal notice sets out the appeal route and the time limit that goes with it. That window is short, and it is written on your letter, so read it today rather than over the weekend.
Apply again with a stronger file. Here you are not arguing with anyone, you are removing the reason they gave. It feels slower, but it is often the cleaner path when the gap in the file was real.
Which one fits depends on an honest answer to one question: was the file actually complete? If you sent everything and they overlooked it, an appeal is arguable. If the sponsorship pack was thin, a fresh application that fixes it is usually the better use of your money.
On your specific reason, with a sibling sponsoring you, what is normally missing is the material tying your brother both to you and to the money: a signed sponsorship letter, his statements covering the period they ask for, proof of where his income comes from, and something documenting that he is your brother. That is the same pack people worked through in the documents thread.
This is the case worth paying for advice on. It is time limited, the deadline is not yours to negotiate, and you realistically get one clean run at an appeal. If you want someone to read the actual wording on your notice: Talk to a lawyer.