Why do some peoples hair look so dirty, dead, and greasy?

My sister in law always has the uglyest looking hair, greasy, dead ,dirty,ect,.....and looks like she never combs it either its always pinned at the back. Its been like that since 5 years ago when i met my boyfriend, i asked him why she never takes care of her hair, and he says she always washes it and it just goes like that??....are some peoples hair just naturally like that???....and why??.....because me and my sisters hair was always shiny, smelled good, looked good, and felt nice and soft, so i never knew that that can happen!!!