In my opinion, people do generally have just one mom and one dad - in the case that one of them is absent, a stepparent might become like a mom or a dad. But if both mom and dad are present in the child's life, those roles should be respected, and the stepparents can become supportive adults in their lives, but not "another mom".

At any rate, becoming "like a mother" to your stepkids will develop organically if it does at all, so just be Stepmom and see what grows from there.

Also, it is different for stepmoms vs. stepdads.  Stepdads are more easily accepted as substitute dads.  You stated, "I come from a divorced family who has a stepdad that is more a father to me than my biofather."  That doesn't surprise me so much.  Generally, stepdas move into the initial family home (that biodad left or was kicked out of) when they take up with BM.  Not much changes.  BM, by far the more psychologically influencial parent and usually the primary custodial parent, is right there, with her kids, backing up and supporting step-dad all the way.  Or, possibly even "coaxing" the kids to think of him as "real" dad.

None of the above usually applies to SMs.  Since bio-dads tend to be thought of as "deadbeat" dads most of the time, whether they are or not, SMs will be thought moreso as the deadbeat's sleeping partner.  Now, granted, I'm somewhat exaggerating here, but it makes the point.  Further, for example, at weddings it is not uncommon for step-dads to walk their SDs down the aisle, while SM may be seperated from any and all of the initial family, incl. her own husband, and told she has to sit in the back and stay out of the way.  Speaks volumes about the hypocirsy and double-standards that exist.

Nonetheless, it is best for stepparents, whether male or female, to stick to the role of supportive adult vs. another mom or dad.  There is too much at risk, especially for SMs, given the above.  Here I've heard story after story about a stepmom who raises a step-child as her own when BM is MIA, only to have BM come back when the child is older and for BM to literally whisk that child away, heart, soul and all.  And, that child may even come to despise SM for taking her away from her "real" mom, even tho. "real" mom may have been a true POS.

Here’s what Wednesday Martin, self-professed SM expert, has to say about it: “. . . Stepmom, research shows, does best when she leaves the parenting to the parents, solidifies her partnership, and takes occasional time away from her husband's kids and even her husband to be with friends and family of her own. This provides relief and rejuvenation, solidifies her sense that she has many successful relationships, and prevents stepmaternal burnout.”

As a fellow step mom I've struggled with this. My SK has also struggled with it. His BM is pretty clear with him that she is his one and only mom, and I am just dating his father. However school has taught him that there are all kinds of complicated families out there. I try to be flexible. I am always clear that I am not taking his mother's place, that she will always be him BM and no one can replace her. Most days he calls me by my name, but some days he likes the idea of having a step mom, or a second mom so he'll call me something new.

Its all about what they are comfortable with. If they decide you are their second mother than thats what you are, it doesn't matter what the BM says. However you can't force it either. Let the SK's decide who you are to them. 

I adore my kids SM. I think she is a wonderful person, I am so happy that she and my exH have a lovely home and strong family unit that our kids are a part of. She includes them in so much and is always thoughtful and mindful of them. --- BUT she is not their mother. I am. 

I am not screaming from the rooftops that they are My ChiLdReN! Or anything like that. It is just simple and legal, that I am the mother, their dad- NOT DH- is their only father. There are times where he (exH) and I will make decisions that have nothing to do with our spouses. It is something that we decide as the actual parents. 

As a stepmother- I get it. I am my husband's wife. I support him with SS15 and for SS20, I am even the primary care giver. I don't even think DH remembers how to set up his medications anymore. BUT, at the end of the day, I am not either of his children's mother. 

Many ways to make up families these days and more than one mom is just fine to me.

Most of us on here though find that in step situations expecting the bio-mother to be ok with another woman "taking her title" is unrealistic and a lot of biomothers are teratorial around things like that. It is best to leave it alone and not push the issue.

Another factor is that coming into a child's life later in the game can mean that the love felt is more fickle from both the child and the step parent than one from a bio or adopted parent. The love is dependent on a good relationship rather than being unconditional. Step parent-child relationships can vary a lot over the years as a child grows and life happenes. Feelings can and willl change. Divorce or death can happen and suddenly the step parent has no part of the step child's life.

It is easier just to establish that step-mom, step-dad are titles are ok and not bad words. There is nothing wrong with them and they are perfectly valid. Being a mom and being a step-mom are different roles and that is ok. Kids have many adults in their lives with different roles who care about them. No need to fight over an issue that can be easily avoided. 

The factual existance of StepParents and adoptive parents prove the BM's in this case is an idiot.

Many kids do only have one father and one mother.  However, nearly as many have more than one mother and/or father.

The law recognizes StepParents as having the authority of in loco parentis and the related duty to care for the safety and well being of SKids when the Skids are with the SParent. The operative part of the title of StepMother or StepFather is .... Mother... or Father.  There is far too much evidence in STalk where a SParent is far more of a mother or father to a Skid than that Skid's bio parent may be.  Guess who the REAL mom or REAL dad are in those cases. It sure as hell isn't the BioParent.

As much as a BioParent wants to claim exclusive status in blended family situations, their position is in fact not exclusive.  At least not necessarily. Some other father or some other mother parents their child when that child is with the other mother or other father.

BM in your case can delude herself all she wants. She is still an idiot.

She has zero say, authority or influence in how you and their father parent your SO's children in your home.  In your home... you are their mother. Just as when they are in BM's home, their StepFather is their father.

Whether or nor your SKids call you "Mom" is another discussion.

Just my thoughts of course.