Hello, I'd like to help you with your question.
It sounds like their could be two possibilities here. One your sister was abused or traumatized sometime during her childhood, maybe even unknown to you. Abusers often hurt children and part of that hurt is forcing them to keep it a secret. And when a child is abused, they often develop personality issues in order to cope with what they are going through. However when they grow up, this behavior begins to create issues in their relationships with others.
Second, your sister learned her behavior from someone in her life as a child. If your parents acted in a similar way or somehow demonstrated dysfunctional type behavior, she may have learned to act in a similar way.
Either way, your sister's behavior seems to be driven by some need of hers that keeps her focused on herself and not others and their needs. That may be why she is not concerned with how other family members react to her.
Your family may want to try these ideas:
Try sitting down with her and talking about how they feel. Use "I" statements, such as "I feel bad when you...." It helps keep her from feeling defensive. And this gives everyone a chance to see if they can achieve the same results you did
when you spoke to her.
Consider a family intervention. This is when all family members get together, preferably with a therapist, and gently confront the person with what they are doing. It is highly suggested that a therapist is used so the situation remains fair to everyone and no one ends up feeling hurt. And it also increases the chances that it is successful.
Learn more about personality disorders and the effects of abuse and trauma as a child. Here are some resources to help:http://psychology.about.com/od/personalitydisorders/a/personalitydis.htm
Unlocking Your Family Patterns: Finding Freedom From a Hurtful Past by Dave Carder M.A., Earl R. Henslin, John S. Townsend and William Henry Cloud
However, if they try all these things and nothing works, they may need to distance themselves. It is hard to do because they may want that connection, but protecting themselves from getting hurt is most important, especially for the children.
I hope this has helped you,