Hello and Welcome,
Batman20! 
Although I'm sorry you had to find us, I'm really glad you did. It really sounds like you all have been through so much with your SD (step-daughter), and I can understand being fearful of how her behavior will impact her half-sister. I want you to know, firstly, that you are not alone and also that things can get better. You've found a great place for support and learning about BPD. Welcome to the Family!
By reading other members' posts, you'll soon find you are in good company. We all are learning how to navigate these intense relationships and helping each other along the way. The site has many clinically responsible articles that I trust more than anything I've read on the internet about BPD. In fact, I rejected my DD25's (darling daughter, age 25) diagnosis after reading all the doom and gloom on the subject.
A good place to start is the thread pinned to the top of this discussion board,
HOW TO GET THE MOST OUT OF THIS SITE. It has many of our best articles with links to more. I also encourage you to read and post in other's threads, doing so will help build your support network, so critical to those of us who love a pwBPD (person with BPD).
Your H's (husband's) reluctance to learn about BPD may be what you say, fear of the reality of the situation. I was afraid as well and only came to accept my DD's diagnosis after finding a therapist who has experience with pwBPD. I wish I had found bpdfamily sooner. Perhaps, in time, as you settle in and begin applying some of the tools we are learning, your H will want to know more.
You say your SD is suddenly showing signs of some disorder, can you tell us a little more about that? What behaviors have cropped up suddenly? It helps to know more so that we can best advise you. Please share what you are comfortable sharing here, there are no judgments, we understand better than anyone else can.
What sort of contact do you have now with your SD?
I look forward to getting to know you and how best to support you.
~ OH