How can I help a parent in need... At my age of 16?

My friend's mother has three children, including said friend. Said friend is 14, and she also has fraternal twins age 10. My friend has some psychological issues, but since being my friend, has gotten better about them. Her problems are mostly physiological. She has PMDD, much like I did, but she wasn't learning to cope correctly. I can help her out here, giving her tools to control herself. She's on Prozac, which I sometimes feel isn't helping very well. But I'm not a doctor, and I'm not going to say anything.

The girl twin, however, has some very serious behavior problems. She throws fits, calls her parents names, gets violent and destructive, and lately refuses to go to school.
Her mother is ill-equipped. The mother has physical problems of her own, but regardless, usually wants "quick-fixes." I have no doubt she would medicate this child when what she really needs is parenting.

I may be young, but I'm pretty good with kids (though I can't say I really like them). Even when it comes to discipline. This child has identified me as a friend, but she also knows I have more strength and patience than her mother if she acts up.

Unfortunately there are lessons she hasn't been taught that make parenting her at this age difficult. She doesn't understand that her actions have consequences, nor that she is in control of her behavior and her treatment.

This makes going to my friend's house very hard for me. I can't stand to watch this child be ruined by this lack of discipline. While I wouldn't mind going there every day (I'm confident that given time, I could break her of these habits), I know I can't do anything so long as this mother finds no fault in her parenting. Plus, even if she WERE willing to take some help in dealing with this child, it wouldn't come from me. People generally don't give 16 year olds credibility when it comes to parenting.

I'm trying to help this family through encouraging my friend to be a good big sister, but is there any other way to give aid to a household that really needs it?