First off, mazal tov on your brother.
Secondly,make up some ridiculously funny story, like training to become an international kangaroo surfing champion. Then after they laugh, change the subject and ask about the other person (like other people suggested).
If they still persistent, try to act normal and tell them that you are in the process of deciding and would rather not explain. Most people would stop asking at that point, but if they don’t understand then you should start acting like a mime and build glass walls around them.