My brother's wife did the same type of thing - she was one person to my brother's family until they got married, then the true colors came out. The issue though, was not her - the issue was him.

In your case, the issue is the father. Sure, the new wife is a manipulative and deceptive b word but ... what is Dad doing? He needs to put his foot down and explain to the wife that his child will always be his child and that, right now, her needs have to come first. Because, you know, she's a child?

Dad needs to grow some balls.

Is there another family member, maybe a grandparent or favorite aunt, you can take Dad aside and explain how this situation is affecting Emma?

As Mom, your priority is Emma. You can use this situation to teach important life lessons - how to deal with crappy people without becoming crappy yourself. At the same time, if the situation is just causing Emma too much stress and tension - maybe she doesn't need to be visiting Dad anymore. What is best for Emma?

Hopefully Emma is getting some counseling to help her process this.