What do I do about my lazy co workers?

I work as a hostess/cashier for a franchise restaurant. My job requires me to do more than hostess and cashier but my fellow coworkers who have been there longer than me just stand and talk. I have mentioned this to my manager yet he doesn't do anything about it. He has complimented me on my job performance many times, yet on occasion he sent me home after the rush "so the other hostesses could do something instead of stand". My istant manager would like me to take the initiative and delegate job duties to the (younger) hostesses who have been there longer. I don't think they will want to listen to me when they've been getting by doing as little as possible. My istant manager thinks that since I'm older, they will look up to me and listen but I disagree. Before I start delegating, may I have your input? Have you ever been in such a situation? Thank you.