I would think that after having been raised in such a stable atmosphere, it would make you more capable of dealing with life's trials, not less, because you have a better emotional foundation. It wasn't as if her parents were switching partners every 5 years, so she's been on this emotional roller coaster all her life. The fact that her dad has had only one major life change, and it's been 10 years or so since then, I think the given time frame does qualify as enough time for mourning the divorce of her parents to call it quits by now, in my opinion. (Of course , opinions are like..... we all have 'em.)

Also, the fact that there is less required of her, that she's not having to blend her everyday existence into a new family , I would think make it easier on her to adjust. She doesn't have to live with her stepmother. She just has to tolerate her for the occasional visit.

While I normally advocate giving greater sensitivity to the struggles of people, I think the stepdaughter is suffering disproportionate to her circumstances. So it doesn't surprise me that some would consider there's a mental disorder here. Though I tend to agree with what others have said on here, I wouldn't go around throwing that suggestion out without giving it a great deal of thought, because I'm not even close to being a qualified psychologist to make such a judgment. So, I'm not going to say that I think the stepdaughter has anything that could be classified as a disorder - but regardless, I think some personal therapy could be helpful for her. Not something I would recommend the stepmom suggesting, for risk of coming off wrong, but the dad might suggest it.