Parenting a 19-year-old stepdaughter living at home

My 19-year-old stepdaughter is driving my wife and me nuts over the past few months. She lived on campus her freshman year, but the $13k+ for living on campus her sophomore year became unaffordable for us (after her mother and I had a baby last year), so she moved home. (I'm sure there's some resentment here.)But what I think is really driving me nuts about her is her choice of friends. She's hanging around a bunch of pot-smoking burnouts who have no motivation in life (early to mid-20s who hang out at a park) and she's dating a senior in high school (also a burnout). And, lo and behold, her grades took a 2-point dive during the fall semester.I know 19 year olds are trying to find their way (God, I remember what it was like being 19 and the mistakes I made), but I feel like she's been so privileged (we live a pretty decent middle-class life) in life that she needs to be better than both her mother and me.We've set boundaries, which she ignores, and have consistently warned her about her friend choices (probably a mistake the way we do it). She also runs away to her dad's whenever we criticize her too much.There are no books on how to parent 19-year-old kids, so I just wanted to see what some of you would do or have done. Or is this just part of being a parent? Whatever it is, it's driving me nuts and I would like some advice. via /r/Parenting http://ift.tt/2rttz4e