Great answer. One thing that can help with tone is to imagine that the person cutting in line might not have done so on purpose - give them the benefit of the doubt. Maybe they didn't realize the line was so long, or maybe they were in line before but needed to go to the bathroom mid-way... You can just let them know in case they didn't intend to be rude. Also +1 to "excuse me" not "I'm sorry" in this case, your sister doesn't have to be sorry about anything. — qoba 1 min ago