I've been married for less than 5 years. There were MANY things I did not realize about my husband aka "H", his previous marriage or his teenage daughter (stepdaughter aka SD) prior to our marriage. He acted very differently. He denies it but I believe, maybe unintentionally, he mislead and kept some secrets. Hindsight is 20/20. 
Ex and he had an unstable marriage from the start. Both participated in lying, screaming, manipulation and victim card tactics. Also, H's parents meddled and had/have zero filter. H had court ordered anger management counseling and treatment for mental health issues during their marriage. Ex used SD as pawn to change H's behavior or get something. Ex often stayed away at her parent's house with SD and gave supervised limited visits to H. SD referred to Ex's dad(granddad) as her true father for awhile after the divorce.  H  received limited visits (every other weekend, 1 day/week, split holidays) and a Social worker supervised visits for several months after the divorce. 
SD wanted to skip visits or leave early prior to me knowing H. H blamed Ex's influence on SD. In hindsight, SD viewed me as a fun novelty during dating and early marriage. I didn't force a "second-mother" on her but did ask for respect as an adult. Our relationship was pretty good and she often hugged me at the end of visits and stopped asking to cancel visits. H has an odd Disneyland and "Mini wife" syndrome type relationship with SD-always has. He never disciplines her (time out, loss of privileges, chores, etc). I am not allowed to correct or set limits besides "Kindly, please don't say or do that again. Thank you." And it happens again, sometimes immediately-lying, stealing, disrespect, exclusion, name-calling, etc. Things we agree she cannot do, H gives permission for her to do when he thinks I am not looking. This is SD's long-standing behavior from early childhood. SD often talks bad about D to me and I kindly defend him. I have tried to love her, include her, give her space, meet her where she was in processing everything, etc. H was happy she wanted to be around more often after I came into the picture. 
Initially, our blended family seemed to be going well until we had our own kids. H said he wanted more kids while dating and married. SD was an only child and grandchild  (center of attention) and has not adjusted to having siblings and cousins (from H's sibling).  Now, she skips visits often and H blames me. (first ex., now me).  Our kids are the hinderance /burden by existing. H literally ignores me and kids when SD is visiting, they have 1:1 conversations or look only at each other like they are on a date. SD has treated me terribly and done many mean things to me. H justifies almost all of them and says she didn't know or mean what she did. SD has asked to have all visits alone with him. I said occasional 1-on-1's, not EVERY (other weekend, 1 day/week and holidays). My kids need a dad too. H works long hours, hangs out with friends alone, etc, so visitation time is really hangout time for the kids. I asked that we include everyone and hangout as a family. How will this effect our kids as they get older knowing daddy wanted to be with SD and they weren't invited? H never talks to me at any meal, driving, etc but when SD shows up he is animated, alive and pleasant to be around. He says it's because he doesn't see her every day but he talks about anything or nothing. He doesn't want to go fun places with me and the kids only when SD is available. So we have to wait around for her to be available. I understand wanting to include her but does that really mean the kids do not get to have memories for the rest of the time?  H is angry he does not have relationship he wants with SD. He treats me terribly, keeps a hate journal of all the things he hates about me or anything i do wrong (no positive journal of me), threatens divorce, lies to me, hides information, gossips to friends, private convos with SD I can't know about.  H lets SD call the shots often and overrule me and disrespect me and kids. H learned a lot of this behavior from his dad. I have discovered the similarities over time, unfortunately, after we were married.
We went to counseling for a while but H refused to comply with Counselor's suggestions. Counselor often sided with me so he privately called her incompetent even though H picked the counselor. He went to her with his Ex too, so he knew what he was getting when he picked her. I stopped wanting to go over time since H treated me worse for days after each session and refused to change/compromise. H told counselor he wanted alone time with SD and she said the same thing, occassionaly, not all the time and if wife is not okay with it, then you need to respect her. Counselor told H to discipline SD and if she wanted to skip visits because she wouldn't follow house-rules to allow her to go her own way for awhile to establish authority. He did none of the suggestions. Now H wants to go again to counseling but only to find someone who will give their blessing with his double-life (2 separate families) scheme. He says I'm selfish for not caring about his needs. 
I don't want a divorce because: I lack biblical grounds and my children need my protection from the craziness. I  don't want my children to have visitations with H, SD or his parents (grandparents) alone. SD is very mean, mocks the kids and tries to undermind my/our parenting. H rarely stops her. H works from home and has refused to put pictures up of our kids, only SD in his office for a long time because it might make SD insecure. He finally relented after a fight I had with him. He only carries SD picture in wallet. His parents (grandparents) say SD is their favorite infront of the kids. They do not have pictures of the kids on their walls even after I have given some to them. They make fun of my kids but none of the other grandkids. H refuses to confront them. I have tired on several occasions to kindly bring it up, they both deny and refuse to change. Grandparents occasionally lie to me and try to teach my kids things against my belief system I have asked them not to teach or give gifts I have said were inappropriate prior to the gifting. Granddad (H's dad) is very vulgar, inappropriate and I worry for their safety if they were alone with him even if others were in the house. H is inattentive when he has watched our kids and they sometimes get injured, minor usually. I can't in good conscious put my kids in this type of environment without me. Alternatively, all of these things (and more issues in our marriage) have lead to many fights between H and me. I worry that our kids will sense this as they get older and I don't want them growing up in that kid of environment either. 
In hindsight, I feel, maybe wrongly, like H  might have married me because he was lonely and wanted to make a "home environment" that would entice my SD  to want to be with him. Since that didn't work, he wants time alone with her and wants to abandon us whenever she is available. I know he desperately loves her and I feel terrible for him that he has the bazaar relationship he has with her. I am also concerned about their time alone together because she has convinced him in the past to spend very large amounts of money on her without my approval. I believe H and I should decide how our funds are spent together. When H is lonely, needs laundry, cooking, etc then we are his fallback. He says that is not true and that he loves me and wants our marriage to work even though he calls our marriage "a living hell". I relocated, sold my house and gave up my dream job to marry H. H did sell SD childhood home to buy a bigger house when we became pregnant, switched churches to make me feel comfortable and some other smaller things. I just don't know what to think anymore. Help me, I need advice.