Hey welcome to the club. Lots of newbies recently. That's awesome. I was a newbie 8 months ago, and I can give you hope by saying that it can get better. This site has been super helpful because of the support of a community that "gets it", and because of all the really helpful information on the site.
If you haven't already found the info, look under "how to get the most out of this site", and the "Community Built Knowledge Base Tools and Skills workshops". There's a ton of information, experience, and stories there which I believe you will find both helpful and validating. Once a person gets reading, it can be hard to stop, because a lot of the stories will fit your situation, and you won't believe that so many people can be experiencing what you have.

The site is also full of strategies to better communicate and manage a relationship with a pwBPD.
Any tips on dealing with this situation in a healthy way?
Can you tell us more or give us an example of a situation with your sibling?