My sister's hair is really unhealthy, but she won't do anything about it. How do I convince her to?

She’s 9, and has hair past her waist. She’s had about a total of 4 inches cut off in her life. She really doesn’t take very good care of it (leaving it loose all day and only washing it once a week). I’ve told her she needs to get it cut or have my mom or myself braid it every day, but she usually cuts me off before I get through it. She says it’s “luxurious”. But it really isn’t. So how do I get her to take better care of it? Everybody we know thinks it’s ugly.