"Going back and forth to different environments with different rules and expectations makes it very difficult to install good behaviors and values in my kids lives."
This is exactly what we parent our kids to expect.
This is real life.
No two places have the same rules and expectations. So unless you can lock your kids up in the house for the rest of your life (NOT!), you have to parent with this in mind.
In the same way, your kids are ALWAYS going to be running in to people whose opinions and methods are contrary to your own. Parent with respect to that. Ensure that your words and actions are congruent. That your kids always see you has honest and loving. They will SEE your words and actions are congruent and make their own opinions.
She can't turn your kids against you. Trust me on that. Only you can do that with your own actions. When my parents divorced, I was 12. My dad called my mom a rude name in front of me. Mom wasn't there..and even though I thought they were both at fault in ways, I still loved them both. I got right into his face and said "DON'T YOU EVER TALK ABOUT MY MOM LIKE THAT!"
Don't worry about it so much. It doesn't matter what the stepmom says. Kids believe what they SEE.