I posted this as a comment on a forum posting and then decided I wanted to share my thoughts more broadly on "how to be a perfect stepmother". Would love to hear what others think...

TL;dr - The secret to being a perfect stepmom is to realize that you have no needs, you don't matter. You exist to make up for all the mistakes that DH and BM made. You also exist to be the scapegoat for all the mistakes DH and BM continue to make. 

1) Ingratiate yourself to BM. Do whatever she wants. Let her make decisions for your household. She's a "mother" after all and she "knows what's best." If she insults you or your DH or tries to have a secret relationship with your DH, just accept it all with a smile. Don't you know that she's the real victim here?!

2) Allow your Skids to do whatever they want. Family decisions to be made? Let the Skids make them. Chores needing to be done around the house? Don't think about asking the Skids. Make sure you cater to them, pick up after them, keep them entertained, provide food and shelter for them, regardless of their age (and especially if they are adults). Make sure you get the food they like and they have the best room in your home. SKids need money? Give it up without question. Don't expect anything in return. They are victims and their trauma makes it hard to appreciate others. 

3) Allow DH's family to meddle as much as they want. They are only looking out for the children after all and just like BM, "they know what's best." Also, you will never be considered part of the family. You are interloper and only trying to take attention away from the children. They have no choice, they have to protect those poor traumatized children (again, regardless of their age). They will always see BM as the "true wife" of  your DH. They will also feel that it's their job to "get involved" and "fix things" between your DH and BM and to "save" the poor children of divorce from the horrors of a broken home. They will constantly be on the lookout for evidence that you don't care enough or do enough for the children...and they will find that evidence, even if there is none. 

4) Now, on to DH. Don't expect him to be your husband all of the time. When SKids are around, he will likely be absent and unresponsive to you. You will be essentially single during that time. DH will likely forget to to important things for you (for example, DH once forgot to pick me up at the airport because he was with his kids) and who can blame him, his precious kids are around. He will feel guilty over getting divorced and allow his kids to do whatever they want whenever they are in your home. He will also want to lavish them with gifts and outings, even if a large portion of his income is already going into BM's pocket. He will expect you to make up the difference. Also, he will want you to "love his children as if they are your own", even if they are "unlovable". You are a bad person if you dislike his children...even if they would be deemed unlikeable by all accounts. DH is a victim, too...he's just stuck in the middle. You need to see that and make sure your needs don't weigh on him.