Hello GratefulforHelp, welcome to the site. You're definitely in the right place for learning new ways of coping when a family member has a PD. These aren't just "normal" difficulties, they're incredibly emotionally intense. Glad you decided to reach out for more support.
It is SO difficult when we're dealing not just with an adult with BPD, but when there are kids involved, and we want to support and protect the kids. pwBPD (persons with BPD) often see their children as extensions of themselves, and find it nearly impossible to tolerate the "gray area" that children can love many people, not just one (the pwBPD). It's possible that your sister may see her son liking you as a rejection of her -- that is an example of "black and white thinking".
Has anyone else dealt with this extreme paranoia from someone with BPD?
Yes, there are lots of members here who have a pwBPD in their lives with paranoid traits and behaviors. Suspicion, jealousy, blame, snooping, and irrational theories or plans are some of the many ways paranoia can show up.
As far as you can tell, does her paranoia show up most strongly in relation to wanting to hurt/kill herself? Or does it show up in other areas of life, too?
Feeling sad and helpless in the face of this situation makes a lot of sense, especially as you see your innocent nephew swept up in her intense emotions.
One place we can start to "do something" about the situation (because, unfortunately, we can't change others), is for us to learn some new tools and skills for identifying dynamics and deciding if we want to change how we interact. One dynamic that can show up between siblings is "Fear, Obligation, and Guilt" seeping into interactions. Check out our article on
Emotional Blackmail and FOG and let us know if any part of it stands out to you.
Most importantly, you are very not alone. So many members here have a sibling with strong PD traits. It's a huge challenge, and at the same time, it's possible to make the relationship "less worse".
Keep us posted on how things have been going;
kells76