My little sister has BPD, and a year ago mum kicked her out because trying to deal with it was giving making it hard to deal with her own depression. 

I moved her down to where I live, in a different city, and have been a kind of proxy-carer for her since then. 

The thing that I really really struggle with is communication. I am a very calm, level-headed and articulate person, and I've always been the mediator in my family. But she flares up whenever we try to have a difficult conversation and it is nearly impossible for me to have effective communication with her about certain important things.

Eg. I have been seeing someone and its getting serious. Our dad is coming to visit and I want him to meet her, but my sister insists that she cant be in the same room as me and my girlfriend (because of how upset she still is about her previous relationship which ended over a year ago), despite the fact that she lives with a couple. And she says things which make just no sense to me when I try and talk about it.