Just do it. Otherwise, you'll always be stuck as a brother, wishing the man she dates were you. Talk about the surroundings, ask a question, ask for help, state an opinion. Be playful, light-hearted and spontaneous. Show your vulnerability.
Props are natural conversation starters.
They encourage conversation and others will be compelled to start talking to you. Change your behavior from the role of guest to host. You are not the passive person in waiting, but rather the welcome committee. You have two ears and one mouth because you should listen twice as much as you speak. Listening is a true art.