Hi, thanks for your answer and questions for clarification. No, I'm not in a position to talk with her parents or her. I just hate to see her often seeming angry and unhappy instead of joyful and loving. I did mention it to her grandmother, who is often the caretaker for the chidren, ("Nana") and she said I should just ignore it. I also mentioned to her older sister that I thought some of her teasing was a little too strong (i.e. taking a favorite toy away and not giving it back until there are tears) and she said that, "She has to learn to take it."
I don't live in the same city as the family.does, and when I visit, it is usually on holidays and birthdays to the grandparents' home with visits back and forth to children's home. All three children are "model" children at their school, particularly the 10 year old sister.