I'm not a practitioner. Also, gentle reminder to
qualify your own resources.
I suggest you consult with a suitable T or P if you're interested.
Can the replacement caretaker sometimes be non-romantic,
I think yes.
for instance a close family relation of the person with BPD,
I think yes.
and what if that relative may also have similar challenges themself,
I think it depends on the challenges.
how can this complicate how the person with BPD now sees and treats the original romantic caretaker?
My first thought is that if that new relationship stirs the pot, and you're still linked to the pwBPD, it's potentially more for you to deal with. It can also absorb the challenges. So it depends on the relationship and the challenges. In any case, if the pwBPD in your life is an ex SO with little to no links to you, I encourage you to go about detachment and separation. Going through that tends to produce a non that can protect themselves from the challenges you're probably speaking of.