My sister and I (22F, Australian, twins) are both currently in Japan, but both with our own set of circumstances. I've been living in Japan for two years now after getting a job here straight out of university, while my sister has come to Japan last December for what was meant to be a 2 week trip - and has subsequently turned into a two month stay. Now, this usually wouldn't be a problem. She's an adult; she can make her own choices about her life. However, the reason for her stay and her lack or thought, planning, or communication on when (or even if) she'll be going home is putting a very big stress on the relationship between her and me, and her and our mum.
The reason for her visit to Japan was to spend time with her boyfriend (a Japanese man she met when on exchange study the year before last/last year), so I can understand why she would want to stay for a reasonably long time. Being in an long distance relationship and finally being able to see each other after a year no doubt makes it difficult to go home, but she'll have to within the next two weeks because she hasn't yet graduated from university and the last class she must take to complete her courses cannot be done online. She must physically return to the university to complete it. But she's never been fond of university, so it's a toss-up whether she'll even go back for that. And, for the past two months, whenever either our mum or I ask her if she's decided when she'll be going back to Australia she either says 'I don't know yet' or completely ignores our messages.
Now, we don't mind what it is she decides to do. Like I said, it's her choice and we respect that, even though it may make life lonely for our parents or dissapoint them. What we're worried about is that she has no plan at all, or that if she does have an idea of what she wants to do, that she'll completely leave us in the dark and cut off communication with us for some reason before long.
Mum has become frustrated, worried and stressed, and she's told me that she's afraid to ask my sister too much too often because she thinks it might push her away. She's also said that for all she knows my sister may have gotten married to her boyfriend (with whom she's been living these past two months, along with his mother) without telling anyone and will never come back home, which is not only distressing for our mum and I, but also for our dad (our parents are divorced), who can only communicate with us through email (and therefore the frequency with which we talk is much less than with our mum) and will never say whether he's worried or not, though he no doubt is.
How can I convince my sister to talk to our parents or me about what she's planning to do?
Neither our mum or I want to change her thinking or make her do anything she doesn't want to. We just want her to communicate and stop keeping us in the dark about her plans.
Neither our mum nor I have any way to communicate with my sister's boyfriend or family, so asking them is not an option. My sister has only now, after two months, given our mum her boyfriend's address, so getting any more personal information about her boyfriend's family is most probably not possible. Getting information from my sister has always been like pulling teeth.
We haven't always gone straight in with 'what are you doing with your life?' kind of heavy questions, and we aren't asking her every day or anything, far from it. The last time I talked (or tried to) to her about anything specific was something like two weeks ago. And although we've tried the gentle lead-in of 'how are things going?\how are you?' it's mostly met with answers that go nowhere and don't really have any room to wiggle in the intended conversation. This is the problem: the lack of reciprocated communication.
Added due to recommendation from the comments: My sister (as far as I know) is currently supporting herself financially through her own savings. She is not working (she can't because of not having the right visa) and she's said (a while ago, so not sure if the situation's changed) that while her BF offers to pay for her expenses she's politely rejected his offers whenever she can. I don't know how much she actually has left in savings though, and she would probably ask our mum or dad for help if she began to run out.