If I had five rules to give to other step mothers, I would let them know that; number one, kids are their own people. Kids are who they are. They are absolutely a combination of your husband and their mother. They are not a replica of you. They are going to move through the world however is comfortable for them. You need to remember that they are their own people.
I would also would remind us that, rule number two, you've got to be able to say "no." You are not super woman. You are a step mom. You don't have to say "yes" to all the soccer games or to every bake sale. You can give yourself a free pass every now and again, in fact, it's critical to do so. That re-engages your energy, so that when you are present, you are present.
Number three is that your kids are going to be like their mom. When they do something that reminds you of her, you need to keep that to yourself. You need to always be respecting their mom. No matter what the circumstances, you have to hold her to a high regard.
Number four, you must take time for yourself. You need time out. You need free passes. You need to reinvigorate. Don't be afraid to take that time for yourself.
The last rule, number five, don't waste any of your energy on the way your husband's ex acts. No matter how she treats you, you have limited energy. You want to take that energy and use it toward something positive;, and that's to be the best step mom you can be. Don't look for validation from her. Look for that in your husband and your kids.