Hello nitasmom,
I'd like to join Turkish in welcoming you to our family.

I am so sorry you are dealing with the pain and fear of watching your Step-daughter rage and mistreat her baby. My heart goes out to you and your grandchild. Such a sad situation!
Along with Turkish, I am also curious if child services is being helpful in your situation. Sometimes "the system" that is designed to help families in distress actually makes it worse, which is frustrating, but not hopeless.
When a child suffers from BPD (even our adult child), not only is the child unhappy and unhealthy, but often, so is everyone who loves them. This mental illness can severely affect everyone, creating drama and heartbreak, while also piling on the guilt and anxiety. The good news is that there are answers to these problems, and we are here to offer you the support and encouragement to help you find them. You'll see that there are things that can be done to stop making things worse and begin to make them better.
Turkish already gave you a link to the lessons for parents, which is a great place to get yourself educated about BPD and what YOU can do to improve the situation by not making it worse. I'd also like yo invite you to post on what we call the "Parenting board."
Parenting a Son or Daughter Suffering from BPD board and hope you join us in learning how to understand and communicate with our children better.
There are several step-parents and grandparents there, along with those of us like me, who are dealing with a teenagers with BPD. (My daughter is 17.) We all support each other because we can all understand what it is really like to live with someone with BPD. You are not alone!