Oh boy. Here we go again. Heavenlike is here to tell us how it is!

Whether it is telling other posters what kind of weddings they may have, or what their home should look like, she's got such an egotistical view of her own opinions that she can't even tell that they are merely opinions.

"Do you understand what I'm saying?"

I don't. I'd like to first see the paperwork that was issued when you were granted authority over defining others' roles before I have any desire to understand your extremely rigid stance.

You are not a Mommy substitute. The parents are split up. That means each parent is now fully responsible for all parenting tasks in their own home. Everything to do with caring for his daughter is now your husband's full and complete responsibility. Always think to yourself, "what would Dad do if I wasn't here" and then Dad needs to do that. You can choose to assist with tasks if you want but you are under no obligation whatsoever to do anything for the child.

"You aren't the other Mommy. If you provide childcare to your husband's child, you are Assistant Daddy.

The term stepmother only applies if the mother is dead or somehow truly unable to care for her child.

Let me guess - you're a BM whose ex has happily moved on and you are severely threatened by the presence of another female maternal energy in your child's life? And so you are projecting your dysfunction by declaring all stepmothers invisible and shoving your definitions down others' throats?

I agree that a stepparent doesn't need to be a parent, and instead should focus on helping his/her spouse out with parenting. But I am not "Assistant Daddy". I am female role model in my skids' lives when they are with their dad. And I think that every stepfamily gets to create their own definition of what works for them, and what works for them does not require you to sign off on it.

Our BM is (quite unfortunately) alive and kicking and I'm Stepmom! Yup! "Mom" is right there in the title and I don't even have any kids of my own! I give their dad advice about how they are parented, and he listens to me. LOL.

This OP is trying to take over parenting the child and is trying to forge a relationship with a BM who clearly doesn't want that and those things are going badly for her because she is approaching step-parenting from a very naive stance.

Those are the issues here, not whether she defines herself as stepmom or "Assistant Dad." If she and her husband feel that she should be acting in a parenting role in their home, so be it. She'll experience the joys and the downfalls and decide whether she wants to keep doing that, regardless of your opinions.

^This.

*stabs eye* CAPITAL!!!!

and hey, i rarely use capital letters }:) }:) }:) }:) i know it kills LF too!
spelling and grammar errors are what really irk me - no offense!! i really should've been a proofreader.

As other posters said "mum mum" is not a good name for you. Too close to mom and it doesn't matter that the child thought it up, pick something else. Use your first name like we did with my steps that were 5 and 9 when I met their dad.

You are presenting this as a problem that is yours when it isn't. The CO-PARENT issue is solely between your DH and his EX. You can be supportive of your DH in being a parent, you can be a positive force in the girl's life and you may even be able to provide guidance and reprimands when she is with you, but you are NOT a Co-parent of this child.

Also, a sunburned child isn't the end of the world! I have had plenty of sunburns and so have most kids I have ever known. Sunscreen can be applied but wear off or not be applied exactly right and not be effective.

I am sure that the EX is an EX for a reason. The good thing is that as the SM, you are not required to interact with her, so don't. Let your DH, who made the mistake of breeding with this woman, deal with her and anything to do with the child they created together.

It's great that you love your SD. I love both of mine too and like to think that I had a hand in them growing up to be lovely girls with good jobs etc... But, I did not get into the in between parenting with my DH's EX. She is a toxic viper of a woman and I let him do all of that. They had the kids together so they get to figure out how to navigate the stuff that is more complicated now that they are separate.

Does this child impact your life? yes. Does the EX do things that are aggravating? Apparently so. Unfortunately, you are not going to win by trying to insert yourself into the parenting mix with this lady. Leave it to your DH and support him by supporting his parenting decisions.