I'm not going to let my sibling go to disney land for his 21st birthday
What is your reason for not wanting him to go.
I can't stand him and he is a spoilt brat.
Do you think preventing him from going is going to fix his attitude issues?  I don't think so, and regardless it's wrong to prevent your siblings from doing something enjoyable on their birthday.  This is particularly true on a birthday as important as their 21st.
WTF. He's going to get what's coming to him.
Your brother being spoiled is a much larger issue that is not going to be fixed by just keeping him from Disney land. The only way he will learn to not act spoiled is to have to earn his own things so he understands the value of what his parents get for him. He also will need to develop a work ethic.