Hi momo!

That sounds like you have a lot on your plate right now. Running a business and financially supporting/caretaking a person with BPD (and her daughter) is a huge load to carry. Are you able to do anything for self-care? Hobbies? Spending time with friends and family? Anything to refill your cup?
Blame-shifting is, unfortunately, not uncommon in BPD. Sometimes it's projection and a reaction to shame. They have their own behavior and they don't like it or are embarrassed by it but because of their intense emotions and poor emotional regulation, they can't handle it. So, they put it off onto someone else.
I can't approach the issue of her assiting in paying as she feels I am taking advantage of her.
I'm sure you see the irony in this. It sounds to me like you're being taken advantage of across the board and you're reaching your breaking point -- which is understandable. Regardless, this is a situation that can't continue as it is. Your own health and well-being won't allow it.
As it stands now, you're enabling both her and her daughter. The best thing for everyone will be for you to gradually step away from that role. Be loving, kind, empathetic, but step away from the role of "rescuer." And be firm about it. It will likely take a LOT of finesse and careful communication. I'm sure others here can step in and help better than I can.
It may mean starting by insisting (again, kindly but firmly) that she start contributing some of her insurance money to the support of the household.
But, to start off, you might want to take a look at these:
https://bpdfamily.com/content/setting-boundarieshttps://bpdfamily.com/message_board/index.php?topic=95263.0