I agree that perhaps making a big issue of it, might not be a good idea, however, when are teens supposed to learn appropriate behavior? I see so many teens who just have no sense of what is proper conduct. They have no idea of what is polite, what is considered rude or what their responsibility is as a teen. If no one explains it to them, they will just grow into an adult with those traits. I see plenty of those too. Maybe this could be a good teaching opportunity.
Many kids don't even know to say thank you when they get a gift or card. They don't even know that it's impolite to ignore someone who enters the room and refuses to speak to them. They think it's fine to sit playing on their phone during an entire meal. Your little-sister may think what she did with the food and money was fine. She won't know any different unless it's explained to her.