Hi and welcome. I am glad you found us. As
zachira said, there are a few people who are in a similar situation with an elderly BPD mother. My parents were somewhat similar to how yours sound. My mother was undiagnosed BPD / uBPD and my father was in the early stages of dementia (though i did not know it at the time).
The last several years of their lives, I had very limited contact with either of them even though I lived only about twenty minutes away so I have little practical advice for you. I think the advice
zachira gave here is excellent. Definitely document and get professionals involved if you can. My mother was the more functional one in terms of getting daily tasks done or ordering my father to do them (she was sick with a re-occurance of cancer and so was weak and tired a lot). In some ways they got closer though prior to that they despised each other. So they sort of banded together in defiance against me (my brother told me).
All I can recommend is to ask questions here and post.
You may find this thread helpful:
https://bpdfamily.com/message_board/index.php?topic=326657.0;all but there are plenty people posting here who are dealing with an elderly BPD parent and all that entails.
Just keep reading as you can and posting to. Ask questions etc.