Sorry - thought that i was saving for later instead of posting. I think this question deserves a little bit of backstory. So yes, we are a unique blended family. We practically have triplets, all with their unique personalities:) Good things are I don't have to deal with a DM and I actually get along great with my Ex so no issues there. But my Sd's defiant, rude behavior is driving me insane and it seems to be getting worse as she gets older. Not only that but I see a lot of bullying towards my children and Im just not sure if this is a healthy situation for anyone at this point. I suppose this situation could be dealt with if my DH and I were a "united" front on parenting or if he would just follow through but he doesn't. My bio girls are not perfect but they at least show compassion and people like to be around them. My ex and I have always been on the same page with regards to discipline so their expectations are no different at his home than here in our home. SD has got DH wrapped around her finger. She is constantly testing boundaries, mean, negative, and starting to show a lot of manipulative behaviors even at this young age.
my DH and his parents, I believe, hold a lot of guilt that her mother is no longer in the picture and don't discipline like I would.

I agree but how does the disengagement process work if I am usually the one with her all the time (besides school).? I think she might be too young and I probably should get out while I can. This has caused sooo much tension within our family. And it's really unfortunate because I have had to be disciplinarian from the start which prevented us establishing a healthy bond in beginning. And then I'm also worried about how it will affect SD if another mother figure "abandons" her.
And believe me, I have talked with my DH about this, and it's like talking to a Brick wall. We have been through tons of therapy with virtually no progress, even our Christian counselor is stumped. I know the problem lies in my DH and not in my SD ultimately. Disengagement is my only option besides divorce. I can't be the only one who cares to do anything. My DH and his parents are in complete denial about how serious this situation is despite many warnings from doctors. She actually has mild brain damage from when her mother did drugs in utero. Another big reason the guilt comes in. They stress consistency and follow through, they also recommend daily cognitive therapy and social skills classes. My DH acts like she doesn't have any issues. If we don't acknowledge it, it's not there kind of thing. I can't live on planet denial , don't know the language. I see this adversely affecting my biokids and it's breaking my heart and dealing with day to day is driving me crazy.

Its becoming clearer and clearer what I have to do in this situation, but I am still torn. I feel like a divorce needs to be thought out and planned for (financially, logistically) and so I am praying that either a miracle is going to happen, or that I will receive the patience to deal with this the next 4-6 months as opposed to just walking out.
My biggest struggle is that this is my second marriage and I promised for better or worse. I do believe that God put me in this child's life for a reason and as bad as it is now, I think she would be way worse if I had never intervened. Now I also want to state that SD can sometimes be pleasant and polite, she sometimes draws photos writing how much she loves me (writes mom in the drawings but never calls me mom), and often talks sweetly about her sisters to my DH and his parents. Bi-polar or early signs of manipulation?!
This behavior is why my DH finds it hard to believe that she is malicious, that and it's HIS child. He says sisters fight, its normal to have sibling rivalry. but I see it for sure and so does our counselor. The more I make it an issue, the more I am blamed for being sensitive, being too rigid with rules, and playing favorites. However, I don't expect anything more or less than I wouldn't expect from my bios. She acts out, lies, sneaks, talks back, complains, or just plain ignores me on an hourly basis. I even have friends that will not bring their children (same age) if SD is around.
I also feel like jealousy plays a big part in her behavior. She is jealous of my daughters because they are extremely close, they have many friends, and genuinely love each other. When together SD constantly tries to leave one of them out. and sometimes because of her behavior, she is left out on things. Even though it has been explained many time, she doesn't grasp consequences. She firmly believes that it is my DH or myself who excludes her from stuff and not the result of her behavior. She plays the victim role, everyone is out to get her and so mean to her. And that SHE is the victim of bullying from my bios. and I think DH believes her half the time.
The last thing that is extremely concerning to me is that SD acts different in almost every situation. She is very anti-social and quiet and school (she is classified as Emotionally disabled because of her past problems with school), very bossy with her stepsisters, somewhat compliant when I am the only one around, Needy and whiny around her father, and totally reverts back to a 4 year old with her grandparents! I know this little girl needs help and she probably needs a lot of love from me. But after the crap she has put my children through and the manipulation, i just don't have it in me! I don't even like to be around her! And I feel like ultimately this is eating at my soul as a person. I don't want to be angry all the time! I think it takes a very special person to take care for a child that's not your own in any capacity and we all should give ourselves a pat on the back.
I really feel sorry for my SD, the DH will realize soon enough.