I purchased an iPad from my brother and he forgot to sign out of his account, how do I sign into my iCloud?

Ideally, your brother will follow this procedure to correctly prepare the iPad for its new owner:

What to do before you sell, give away or trade in your iPhone, iPad or iPod touch – Apple Support

Following this procedure will properly sign-out of all services associated with your Brother’s AppleID, erase his sensitive information - and, critically, removes the Activation Lock from the device.

He needs to sign out before you can sign in.

show him -> How to remove Activation Lock - Apple Support