It sounds like you are doing a great job. I have a 14yo stepson and we have 2 other children together. Some things that have made it easier for me: I had to come to the realization that the kids are not 'equal', once I accepted that it made it easier. I realized that my stepson is probably always going to try or at least wish that I was not in the picture and his 'real' parents would get together again. I backed off totally on any parenting duites besides regular support, cleaning, cooking, clothes, rides, I no longer tell him to chew with his mouth closed or to not lie down on the table while eating. I only rarely will ask him to do anything like pick up his shoes, or help empty the dishwasher. He questions every decision I make, takes issue with the the kind of sunblock I use on the little ones, etc. I bite my tongue and do not engage, he wants a fight and he wants to see my husband and I fight. As hard as it is, the less I engage the easier our relationship is.
I think the responses you got about holding her up to higher standards don't live with a step. My ss can bring down the room and the whole house for the whole weekend if he doesn't get his way. Now I can say that's his problem and his parents problem and his future counselor and wifes problem!!
So I mostly mind my own business. the book that helped me is 'how to talk so kids will listen and listen so kids will talk' as well as 'mom, get out of my alife, but first take cheryl and me to the mall'.
The less 'work' I do trying to make a harmonious family, the better. I do talk to my husband, but he also does not see what I see, so I don't talk about it anymore. The problems just aren't there like they were when I was banging my head against the wall everyday. It is getting easier now that his social life has started, he is not going to identify with his family as much in his teen years. I think if you can make it through the next few years, tweens are difficult, it will get easier. I lowered my expectations and I am much happier for it. He has a mother and I will not expect him to appreciate anything that I do, it works much better.
Someone described step parenting to being an unpaid babysitter, frankly babysitters get treated better. Keep your chin up, stop trying so hard and enjoy your family.