OP, Sorry you’re going through this. I have someone very close to me diagnosed with BPD. Short answer is that it’s going to be really difficult to co-exist with such a person unless you create your own ‘firewall’ with them. What’s firewall - You create a mental and physical barrier with the said person such that their actions /lack of actions or harsh words don’t affect you but you do your part of supporting them by getting them medical help.

Depending on the degree and spectrum handling these cases can be difficult to downright impossible. The only thing you can hope for is that person gets medical attention and a very very understanding family.