Are your 4 and 6 year olds from the same father? If not maybe you resent the 19 year old because she's a product of your ex, who you clearly resent.
Oh, c'mon....where did you get "resent?" I haven't heard her say that. She seems to be being honest about his lack of being able to (or wanting to) set rules for his daughter, not something unusual with guys and their adolescent daughters.