Stepmothers are in such an unenviable position that before one starts to issue advice, you have to go back to basics and look at the actual word. Nothing wrong with "step" – but "mother?" All children have mothers of their own and, even if they died when they gave birth, no one can replace them. Small wonder that the word "wicked" is so often affixed before the word – it's done to eliminate any idea of this new woman in the family has anything at all to do with the real mother.
So the first thing a person in this position should do is to eliminate the word. She can be the child's "auntie" if she likes – though preferable for her to be called by her first name. That keeps the boundaries absolutely clear. She should never introduce herself as a stepmother but rather as the child's father's new wife or partner.
Secondly, this new person should never assume any authority over the children that she hasn't already agreed with the father and she shouldn't really treat the children in any way other than she should treat her friends' children. And she should remember that it is not up to the father and children to welcome her into the family and make her feel at home. It is up to her to ingratiate herself into a family that already exists.
Thirdly, she must remember the children have probably received a dreadful blow, with their parents separating. Their default position will inevitably be one of hostility rather than welcome because it seems to them that she is an intruder in their lives – as, indeed, she is.
Finally, she must remember that her husband's duty is, rightly, firstly to his children – not to her. She must make it clear that not only does she understand he loves his children more than her but that he should love his children more than her.
I'm being hard, I know – but acknowledging those raw truths will help everyone in the end. It is she who must do all the work, bend herself to the family's ways and, if she wants to make life easier, then it is she who must try to forge friendships with the children separately, in her own right, not just tagging along on jaunts out with dad.
And if she can, she should, very slowly, try to form some kind of good relationship with the mother of the children, even if she finds it difficult. Being friendly and reasonable will make it more difficult for the woman to pour poison into the kids' ears.
But in the end, if the intruder is not only lucky but also works very hard at making the new little people in her life into friends, she might well end up with a very special role – someone who can see a situation more clearly than a parent, someone who might be able to offer support and stability in a situation in which the parents are locked in dispute.
But I'm afraid, the "wicked" comes with the job. It's a given. It is up to her to her to behave in a way that makes the word completely redundant and hope that, in time, if she simply becomes a very good friend to the children she'll have done a really good job.