Things ive learned that has helped MY sanity
1. Don't view your step as a child of yours
2. Don't have expectations of children that are not yours
3. Don't make rules for children that are not yours
4. Don't discipline children that are not yours
5. If there is something bothering you anout children theat are not yours, bring it up to your SO in a loving concerned manner.
6. Come here to vent about the annoying child in your home theat is not yours.
In following these rules, life has gotten less stressful, I used to view sd as a child of my own and that her bad behavior was my problem, that somehow it made me look like a bad parent. Then I realized that sd behavior wasn't my concern. It is still pretty stressful, esp when she's throwing accusations around about me or my kids, but it is significantly better than it was before, I was having serious panic attacks eowe trying to very her behavior under control... now it's DH's job, and our relationship has gotten so much better.