Hi JosephMerrill
You're in a difficult situation here unfortunately. Dealing with someone who has BPD can be quite challenging and indeed take a toll on you. Has your sister in law been officially diagnosed with BPD? Has she ever received any kind of treatment or therapy to help her deal with her issues?
No matter how you decide to move forward, I think you might benefit from taking a look at some resources about communication techniques:
Communication Skills - ValidationEnding the Cycle of ConflictThese communication techniques might help you talk to your sister in law. You say she can't work, is that because of her BPD?
Take care