Usually I just lurk on this board, because I don’t like to register for anything. However, your situation brings back so many memories that it prompted me to create a profile and respond. (Be warned, this is a bit long). Both I and my FH are both Vietnamese. My Future Mother-In-Law, bless her, is nothing what you describe. However, my own mother is a terror like no other (well a bit like yours). Growing up it was hell. Even though I had eight siblings, my mother did love to mentally torture us all. How she found the time with a full time job is beyond me. I’ve dealt with the same suicide threats, the fake illnesses, the entitlement issues, the cultural concept of “I’m your mother, I will always be right.” My entire life she has manipulated anything and everything to suit her purposes. This is my advice to you, having been there, you need to leave.
I’m not saying you have to abandon your boyfriend. However, you do have to start thinking of your own sense of well-being. This is not a healthy environment for you. Your Future Mother-In-Law knows she can manipulate you. She has done so many times, and you only stood up to her once. Not that this is your fault. Clearly the woman has deeply rooted issues. I suggest seperating yourself from the situation at least temporarily. Continue the relationship with your boyfriend but make it clear you cannot live with her. You’ve been more than patient and understanding. This does many things. It shows your bf that you had enough of it, and he needs to make a decision. At some point, whether he feels he is in the middle or not, he has to make the conscious choice to defend and back his future wife up or risk losing an amazing person in his life (I’m assuming you’re awesome bc you seem to have the patience of a saint). Also, he needs to learn how to stand up to his mother. Otherwise, what life will you two have? You will only have peace when she is dead (not that I’m wishing anyone dead). By then, she would have financially ruined you both (as mentioned by another bee) and have done everything in her power to wreck your relationship. You seem upset now. How many more years are you willing to put up with this? I’m not trying to be cruel. This situation is hard, and it will force you and your man to make even harder decisions.
In the meantime, maybe to help the financial situation, as a fellow bee stated above, traditionally regardless of who your Future Mother-In-Law is living with, all the children should be contributing to your FMIL’s upkeep. You should speak to the future siblings-in-law, via your future husband, that they need to start opening their pockets. If you get enough money, get her own place, and all of the kids can take a turn checking in on her (I’d say move all her things, show her the place, and change the locks to your house, but thats just me). Your man needs to remind them that if he goes broke taking care of her, someone else will have to take her in. It’s amazing what a threat of her moving back in with them will do.
Lastly, I don’t think a letter will help. It will just give her more ammunition. Gives her another piece of you that she can manipulate emotionally. She seems pretty set in her ways. However, if you do decide to go with the letter route. I wish you the best of luck. I can read and write Viet. I’m a bit rusty, but if you need a hand to help, I have two. Good Luck!