Secret wedding help?

Hi, my partner and I have been together for 16 years this June. We have never married and are considering a surprise wedding at a bar my brother owns. We aren’t engaged and aren’t the type of people to have a huge party but we want to invite a lot of family and friends and impress that it’s an important event but not give anything away. The only people that we would tell would be our parents, our best friends (a couple who would be our wedding party) our eldest daughter and my brother (venue owner) What I need help with is how to set up the invitations and what the plan would be or an idea for a party.