Hi Maggie83!
Welcome! I am so glad that you've joined us! Your post already tells me that you have gone through your own difficult journey, like so many of us here. Thank you for sharing with us!
I don't know a lot about dementia and BPD as a combination, but I would say that each one has it's own uniqueness as well as the possibility of crossing over into areas that would make things worse. For example, I know that with dementia it is not unusual to loose a sense of personal boundaries, or limits as to how one acts (no checks and balances to behavior), and with a BPD being sensitive to rejection, I don't think it would be surprising to see the BPD's reaction become worse or more noticeable.
When my uBPDm was in the hospital after surgery to remove a brain tumor, she started showing a greater manifestation of paranoia than she normally did. Obviously her body was under great stress from the cancer and the surgery, and it was pretty clear to see that one impacted the other. Have you done any research about the typical symptoms of dementia? Has she been to see a doctor yet about these concerns you have? That may pose a challenge for you to get her there!
I'm really sorry that you have to deal with this additional possibility of dementia on top of her uBPD. I cannot imagine how hard it must be for you. What are you doing for self care, to help you get through the time you need to spend with her? It sounds like you've had some helpful T in the past.

There may be some other members who can chime in to share from their own experiences too.
Wools