Getting legal advice, like others have suggested, will likely be worth whatever you pay for the consultation.
My workplace offers a "free legal appointments" benefit (you sign up during open enrollment), so you could see if you or your spouse have that benefit.
You can also call a few local lawyers, explain your situation, and see if they will do a free or low-cost "initial consultation". You wouldn't be required to retain the lawyer at that point, you could just get more information about legal requirements in your area.
There are websites like avvo.com that let you sign up for free to post legal questions, and get free answers from lawyers. Check it out to see if it would be a good fit for you. Others may have already asked similar questions, too.
I lean towards 50-50 with the BPD daughter's inheritance being placed in some sort of trust, NOT to be administered by the other child.
Wise idea -- to try to minimize the drama and conflict for your non-BPDd.
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Would your BPDd feel like she "won" if she received, or felt like she received, "more items" than your other D?
For example, maybe your will would have a list in it of "who gets what". While the money split might not be exactly 50/50, perhaps there are items you could list out, such that the list on your BPDd's side is longer than the list on your other D's side. Some pwBPD respond well to "getting more items" than the other person, even if the more valuable (whether tangible or intangible) things go to the non-disordered person.
Maybe your non-BPDd could receive more financially, plus your home, plus other key decisionmaking items, while your BPDd could receive things that would feel "big ticket" to her but would not damage your other D's future (vehicle, household goods, etc).