You are in a challenging situation that benefits each of you. It would cost your mother more to pay someone to do what you are doing for her. If you were to get paid for the tasks you do for her, it could change your Medicaid qualifications.
With someone with BPD/NPD they seek control through whatever power they have. Your mother seems to hold the financial power, while you have the caregiver power. While you may not be thinking along those terms, consider you are not powerless in this situation. In actuality, your mother is dependent on you for the tasks you do- but her NPD aspect means she won't likely acknowlege that.
My BPD/with NPD mother used money as a means of control and power. Also, to avoid the appearance of being dependent and to feel superior, she could be verbally abusive to the people who were helping her the most- all the while being completely dependent on them in her elder years.
She made some poor financial decisions but we had no way to intervene- even for her own behalf. She remained legally competent and so we kids had no input to her decisions. I saw where she had redone her legal papers on time ( that I know of) and they appeared weaponized, so yes, if your mother is legally competent, she could do that.
Ultimately, if your mother is legally competent then, even if it's not fair to you, you can not count on her providing or leaving you anything. Maybe she will, but maybe she won't. On the other hand, should you decide to move out and not help her- she would also be in a difficult situation. She may threaten to kick you out, but if she knows it would be harder without you, she may not do it---- but she won't admit to that. She needs to feel she's in power. Keep that in mind. This may be posturing.
However, you must also protect yourself. This may mean saving some of your income into an emergency fund in case she does kick you out, while still maintaining your eligibility for Medicaid. If you do inherit property or money at some point, that may impact your eligibility. Another aspect of Medicaid that might affect you, is that, if your mother is ever in a nursing home on Medicaid herself and she owns the house- Medicaid will take that to cover the costs. If she transfers it into your name it can affect her eligibilty (Medicaid look back) and possibly yours at some point. If you need to look into this situation further, a local council on elder or elder law attorney can assist you. At some point, your mother may need to assign power of attorney to you if you need to assist her beyond errands and help with medication, and if she is willing to do that.
For now, if this arrangement is working for you financially, I think your game plan is to maintain this win win situation, and keep in mind that you benefit her when she says otherwise. My BPD mother complained, said negative things to me about one of her caregivers, told me several times she was going to fire her. She never did. This woman was wonderful to her and she depended on her.
However, in this situation, you will need to help maintain your own sanity- by doing things on your own, getting therapy, having some "time off" from being your mother's helper. If you did have a way to move out, it would be an option, but from what it appears, it's the better one financially for the both of you.


