Complicated story

I find myself in an impossible situation with my fiancé of over 16 months, having been together longer than this and when not together we were always on Skype.
I have travelled back and forth every two months to USA as long as my visa would allow, In between times we wound travel to Asia, The main thing, we were together, and had so much fun,
She has an x who is the father of her children (18) her daughter who is bipolar and a son of (21) years both are students. Her x husband and I spend money for there upkeep and they both live with there mother. We are never apart for more than 8 weeks and most of my time is spent in America. We have also discussed me relocating to be with her until such time as the children are settled.
Anyway the ex is back on the scene saying, the kids want to go back with him and clearly she does not want to loose contact with them. Her bipolar daughter has threatened her, its going back with him or they don’t want to speak to her anymore. This is harsh, and it’s upsetting, and I don’t know how to cope.
I have always had a close bond with the children, but this is killing me,
I know she still loves me, she has told me many times, but she has said to me she can’t loose her family,
Clearly they want to relocate and they want her to go with them, she has told me she does not want to go really and wants to stay with me but her kids come first.
Her children have always said they want her to be happy and she has been until now, she is a very honest person and if we had problems we would talk them out, She said she will have to go with the children. She said she loves me so much but her children must come first even if it means staying with her violent ex husband. I feel she is being blackmailed by her children and husband.
We are so close, and everyone who knows us just can’t believe she is being put in this impossible position. She has told me she loves me so much but she has to be with her children.
She has asked for some space as everything is getting on top of her, and is taking antidepressants. She has promised to call me in a few days and we will talk. I have heard nothing yet but its only six days.
I am getting depressed and don’t know what to do. I want her to be happy but just feel this is an impossible position for anyone to cope with.
Can someone please help me, what should I say to her when she does call. Should I let her go? Clearly it has to be her choice, but is clearly being blackmailed.

An 18 and a 21 year old are adults. Does their mother need to be with them?

For starters, my heart goes out to you in your situation, as I am in a similar one, (however, my child is 13, and I am in the process of divorcing/relocating out of states).
I have read your post over several times, attempting to make sense of it. And I apologize if I may seem sceptical. I don't mean to question any of what you have stated. (Please know that I am, by no means, questioning her credibility or integrity, as I'm confident that she is overwhelmed with the complications involved). But because of the fact that, as stated above, am personally experiencing a similar situation, (though not in its entirety), I find some of this just not adding up.

It would seem to me that the fact that her children are of legal adult age and are going to school that they are, for the most part seemingly attending to their own selves as individuals, which, of course, is fantastic.
However, the fact that they are of legal age also means that they can make their own decisions?
You have stated that they have expressed to her that they want her to be happy, yet at the same time they are blackmailing her by what may be through the cruelest means by playing on her emotions?
They are telling her to either go along with them, or lose them entirely?

I find this somewhat confusing. Because of the fact that her children are at the age where they can decide for themselves, how then, can their father have such an influence on their own personal decisions to the point where they are forcing her to choose between her own happiness, (which you clearly stated the children are supportive of), and any future contact with them?
And what's harder to understand is the fact that her ex is abusive to her? She is contemplating on returning to the subjection of the very abuse which she previously left? That alone is an extreme action to expect out of any one person to ever decide to resort to (especially having already left it). Confusing.

What about the discussion of the plans of you moving over to the states to be closer to her? Is there any reason why that still cannot become a reality?
Although I understand her requesting the need for a little space due to the pressure, I fail to understand why she is requesting that space from you, as you are clearly supportive and understanding to her dilemma, as well as being a direct inclusion to all of this.
And the primary fact that your relationship has been going on for well over a year and a half, as well as into the seriousness of engagement of marriage, is more than enough to support your means of concern.

Unfortunately, I am in no position to offer you any specific answer to your question of what to say to her when she does call (if she hasn't already).
The best I can offer you is that you go with your instincts on this. Perhaps discuss with her the possibility of your previous plans of moving close to her to provide her the support she apparently needs?
Is letting her go necessary? Why must you feel that you have to let her go? Has she given you the impression that releasing your relationship with her is your only option?

Discuss your options. (Pay note to her willingness of your involvement to all of this).
Go with what you are feeling. Remind her (not that it's necessary) that you love her and that you are willing to do what's necessary to make it work so long as it is still a mutual goal of yours?
Complicated INDEED! And an emotional turmoil even more.

I'm sorry if I couldn't be of any specific help to you. However,
I sincerely do hope the best for you and your love. Keep the faith. And with God willing, all will work out as it is meant to be.