Denver, I live in a step situation. My H has done the same thing to me. I don't believe he would have done it to the kids bio mom.
I backed off of disciplining my steps and left it up to their father. However, I became resentful because he felt too guilty to do much disciplining.
It's so hard to be in a step family. I wish I had the answers for you but the only thing that worked for my marriage was for the stepkids to reach adulthood and finally get their own lives.
M:63 H:53 S:41, SS:28, SS:25, SD:23 M:15 T:16
Bomb:12/17/11, "I think we should go our separate ways." H moves to his mother's house, 4/1/12 12/21/12: H moves back home, piecing