Q: My ex-husband and I have joint custody of our 6-year-old son. He has a new live-in girlfriend, and he wants her to fulfill roles that I feel are inappropriate. I disagree with her "baby-sitting" or bathing my son at this point. I've never met her. He says this is none of my business. What is good ex-etiquette?

A: How much responsibility does the new partner really have? Technically? Absolutely none. But if she's living with your ex and your child, she'll interact with your child on some level.

It's not uncommon for mothers who share custody to be uncomfortable with the influence of another woman, especially if the other woman is cloaked in secrecy. The not knowing who she is, what she stands for, her morals and how she presents life to your child can be unnerving. Fear can make you come off as an irrational harpy, which makes your ex shut you out even more.

Meanwhile, someone you don't know is making decisions for your child and you feel as if that ex is laughing in the corner.

So let's look at what you really have here. If you and your ex are to share custody, you both have to stop being babies. The child's mother should have met the girlfriend before she moved in. Your child is 6. He's a sponge. If you aren't cooperating, if you're setting up communication roadblocks as a way to control the other parent, if you have no plan for your life now that you are divorced, you're hurting your child. The tension that he must endure because Dad and Mom won't cooperate is not good for him. You are teaching him how not to get along.

Mom, you cannot dictate policy at Dad's house, but Dad, being a better communicator about where you see your life going will ease her fears and allow her to act more reasonably. If this new girlfriend isn't a permanent fixture, she shouldn't be there. And, if she is, be better communicators and set clear boundaries. As much as you may both hate the whole idea of dealing with an ex, it's your responsibility to do it as well as you can for the sake of the child in your care. Starting now.