Oh dear. I can imagine who you are talking about and think you're right to get involved. I am sure she is repeating the type of thing that has been said to her, but all the more reason to pick her up on it. Without supportive and consistant core adults she will never learn. It's not her fault life has been tough to her, but it's also not your daughters. The older sister also needs to learn to stand up to her. So yes you're right to get involved.