It's a tough situation for you to be in. My SO's adult D is mentally ill and creates major chaos at the drop of a hat.
I have been with SO nearly 9 yrs now and have so far refused to meet his 2 kids. (Big backstory to that issue) As you can imagine, after 9 yrs. I have MANY stories about D drama. Enough, that I really don't wish to meet her. However, I also know that I can't attempt any relationship with the youngest (still minor) without attempting a relationship with adult D.
Still, I know the drill where she is concerned and feel lost as to what would be the best way to handle her, when (or if) the time comes.

On the one hand, I have sympathy for her b/c I know she is merely a product of nature vs nurture. (A BM with the same personality disorder and absolutely no ability to parent unselfishly... PLUS, A BF who parents in the most guilt ridden manner imaginable.)
On the other hand, she is unable to maintain ANY relationship (outside of immediate family) because of her overly dramatized and chaotic behavior. She will love you to death until she no longer has a need for you. It is a vicious cycle of idealize and devalue. Her destructive and often times violent nature, combined with her manipulative and impressive pathological ability to lie makes her a very scary young woman.
In my everyday world, I would NEVER allow such a destructive and toxic force to enter my personal circle. I have quite strong boundaries and would never allow myself to be bullied and guilted by this young woman.

So, what to do???? Attempt a relationship on MY terms? Should she push my boundaries, (and she will) do I then yank away any previous offer of friendship? Will my backing off leave her feeling abandoned by yet another 'adult' in her life?? Is it even my problem to worry about??
I wish I had the answers. Any answers. I would surely share them with you, lol.

I think you have to ask yourself how much you are willing to accept from her, (behaviorally/emotionally) and let her know right off what is totally unacceptable to you. Then, DO NOT EVER deviate from it. To do so, signals a weakness to these type of people and will make you the target of some pretty nasty battles. I won't even bother speaking of the extra pressure between you and her BF that will no doubt be sparked eventually.

Hang in there and don't be too hard on yourself where she is concerned. Remember keeping a healthy distance, could be a very good thing.

Same here-I figure my ss will actually end up in juvie. I used to worry incessantly about his problems and try to get him some help-his parents weren't on board so what can you do? Nothing.