Okay, so let's go through the important things.
* Do you have any reason to believe her boyfriend might be coercing her or deliberately trying to isolate her from her friends and family?
Not any based on any kind of fact, as I have yet to meet him or talk with him. Any information I have on the guy is completely second hand. I don't know what he and K get up to, as K doesn't talk to anyone.
* Has she shown any other signs of dangerous or risk taking behaviour beyond unprotected sex, like self-harm or binging on alcohol/drugs?
I have not seen any physical evidence of cutting or of her taking substances. However, I cannot say for certain either way because she just plainly isn't around.
If there's no evidence that that's the case then honestly, it doesn't sound that unusual for someone in her position.
To effectively move out, do the closest thing to cutting off all contact? I had phases where I didn't talk to anyone...but not for nine months!
I'm going to guess that what happened is that she fell out with her mother,
Yes, both the mother and K (on the extremely rare chance I get to talk with her) have confirmed this.
who brought you into it (which was a shitty thing to do) and now you're lumped in with her mother as just another person trying to control her life.
I have already considered this myself. To borrow from debate parlance, while it would be a valid argument, it would not be sound (I hope you know the difference between validity and soundness).
However, on the rare occassion I do talk to her, I have stressed as good as I can that I am acting as my own person, that I don't necessarily believe all the things the mother is saying about her (I have bad history with the mother, and K knows this), but that it is very hard for me to take K's side if she shuts me out.
Young women often do have unprotected sex. Often, it's not apathy
I'm not saying that I think her having unprotected sex is due to apathy. When I mentioned apathy before, it's how I perceive her acting towards me. She shuts me out, doesn't even want me around on her 18th birthday, and whenever I bring up just how little I've seen or talked to her this year (considering we spent 4 years apart)...there's not a sign that she cares. "So what?" is something she's said on more than one occassion.
Having to ask for the pill can be an intimidating experience when you're very young and worried about being judged, and a lot of women that age don't feel secure enough to ask their boyfriends to use condoms.
Nothing much I can say to this...
Basically, it sounds like both you and your mother came in from quite a punitive angle,
The mother yes, she's the shouty type...but me? All I did was bring her to my house, sit her down and say (paraphrasing because it was about 9 months ago), in as calm a tone as I can do, that I had talked to the mother on the phone and was it true, and upon confirmation, what precautions she was taking.
At that point, that was basically when she started shutting me out.
However, I will not deny that it is possible that in her mind, the mother and I are somehow acting in concert on this, that we more or less act the same and thus deserve the same treatment from her.
I know it sucks, but sometimes a person has to hit rock bottom before they can see for themselves the harm they're doing to themselves.
It's not just the sex that has both the mother and I on edge about her. See...she basically doesn't have an education worth calling such. As I indicated before, she didn't go through what Americans call high school. She dropped out in her first year. Couldn't handle it.
As of this moment in time, she has no job, and the only thing she is basically doing (at least onstensibly) is college level dancing (which she honestly is very good at, she's performed live on stage before and has won trophies).
Come K's 18th birthday, which is at the end of November, K will be legally an adult. This means that my mother will be charged rent for her (it's a thing with our local housing authority, kids aren't charged separately, but adults are), and my mother doesn't work, she's on social welfare.
The problem with this whole situation though is that with K's silence, there is no documentation to prove K is no longer living in the house. Mother has indicated her extreme reluctance to pay the extra rent for someone who for all intents and purposes is not there. And if she doesn't pay it...well I hope you can imagine what might happen.
So, I'm going to give you the hardest advice, and it's going to make me sound like a dick, but I genuinely do think it's the best thing you can do for everyone. Walk away.
Don't feel bad. It's what my best friend thinks. He loathes my family with a holy intensity because each member (apart from the youngest sister who is 7) has fucked me over royally at one point or another in the past.
Let her know that you'll be there if she wants your help, and then cut yourself off from it.
The problem here is that K is in no way able to survive on her own, once she hits 18. At some point, she and the boyfriend will break up and then what will she do? She'd literally be on the streets and I honestly doubt that she could physically literally survive.
She'd either commit suicide or someone will take advantage of her.
My oldest sister I haven't seen for twenty years, she willingly removed herself from contact. While that is her decision and I respected it...I don't want to have to go through the same anguish all over again.
but in her own mind she's an adult and she's frustrated that people don't treat her like an adult.
The funny thing is...we are! We're telling her all these things about getting a job, turning up to work on time every day, not skipping days (I've been shown proof that she didn't turn up for large portions of her teenage dance classes, which were basically the only structured activity she did during those years), getting paperwork done for grants, rent...basically the things adults have to worry about, but stuff she's shown no interest in resolving.
The issue is, she's not behaving like an adult because she hasn't yet figured out that being an adult means she'll have to deal with the consequences herself. She needs to learn that, and the best way to learn is by realising that other people won't stick around to watch her mess things up.
Mother has already resigned herself to this, but my real fear is that if this continues, she will with a very high likelihood end up dead sometime within the next year or two, if I wash my hands of her.
Thanks for all this advice.