I found out I have a disorder but I'm going to lie about it so my brother never finds out.
Why would you want to lie to your brother about having a disorder?
I feel like if I tell him it will ruin our close relationship
I understand you wanting to make sure your relationship doesn't go sour but lying isn't the way to do it. If he finds out you were lying he might feel he can't trust you or that you don't have faith in him and that would be worse. Lying is wrong and an easy way to ruin any relationship.
There's only a one percent chance that he'll find out I have a disorder. It's not like I'm keeping a secret that hurts him.
Is something happens to you and he didn't know about the disorder it will crush him. Tell him what is going on and he will be there to support you.