Hi Allthewayup!
Welcome! I am really glad you've found our online family. There is a lot of helpful information at our site as you mentioned. Even just reading through other posts you will continue to gain insight. How did you find out about BPD initially?
There is another reason why I am especially glad you've found this site: because you are wanting to be an advocate for your wife and her siblings. Coming from the position of being an adult child survivor of an uBPDm, it is SO huge that you want to help them. You have a good handle on the fact that there is emotional stunting or delays that occur in the normal growth of a child subjected to such an environment. I've been in T for a while, and am currently working through the ages of 15 thru 18. Tough, hopeless years, so I am thankful you are willing to help.

They need to hear that you care. It will be huge to each of them.
I write today to ask you for opinions as to when it is right to intervene when her rages may lead to physical harm to her children
I am going to include a link here for you that has been very helpful to me. It is a workshop addressing the very issues you speak of.
When are the children of a BPD parent at risk?There is a lot of information in the workshop, but it helps answer the questions you've pesented.
The type of behavior you are mentioning is very destructive and abusive as you seem to be well aware. It is not a safe environment for them to live in. Do they live in Germany as well? Does your wife know now that this behavior is typical of BPD or does anyone else in the family know this? What other options might there be for them to find a safe place? Close friends and their families? Relatives?
I'm sure others will pop in and share their thoughts too. It is a very dangerous time for the children, especially given that their mom is encouraging them to commit suicide. They must certainly feel so unloved and devalued. Please keep in touch!
Wools