How can I get my 7 year old brother to stop asking for stuff whenever we go somewhere?

You can't make him stop asking, but you can change how you respond. If you give in when he whines, then he learns that if he whines long enough you will eventually give in. You are teaching him how to respond to you in order to get what he wants. Be unemotional but firm and do not give in to him when he whines.