I'm not sure if you meant when putting the word "boredom" before your feelings, that you think they don't matter. They do. You matter. I also don't know how long you've been stepparenting or if you plan to have bio kids of your own, but it sounds like you have jumped in with both feet as far as embracing being a "bonus mom."

Every situation is different, but finding personal fulfillment in stepparenting long-term is rare. Even a "good" or reasonable BM isn't going to want to share the "Mom" title. She may gladly share the work, but when it comes to decision-making power or glory/accolades, that's something else. I hope your husband supports you and values you for *you* and not just how you can help him with his kids.

You sound like a good person with good intentions. Please take care of yourself. 

While your intentions and feelings are valid, I'm afraid you are setting yourself up for nothing but heartache. These children already have a mother, and she sounds very involved. I'm trying to be gentle here, but your desire to have "mom" experiences with your stepchildren is probably not going to happen - because they have a mother and she is going to resent your efforts to mother her children. I'll be honest, I'm cringing at your use of the word "our" because these kids are not yours. It is great that you seem to have a good relationship with them, but something that can harm that is if you overstep what should be clear boundaries.

In my experience, the most succesful stepmothers take on a role more akin to an aunt, instead of a mother. They have fun experiences with the children and guide them and occasionally parent them - but they keep in mind the kids already have two parents. It sounds like you really want to be a mother, do you and your DH have plans to have your own children?

These kids have a BM.  That BM is all they need. The SK don't want to disrespect there mother by including you.  You are a nice person, nothing else.  First time you try to control them you will get " your not my mother " story.
 

 If you want to be a mother get a kid. That plan and simple.   DH had kids with BM. If he loves you he will have kids with you.  If he's against that he doesn't love you.  You are not important to him.   I know it's not simple.  But if this is wgat you want you must somehow do it. 

I'm going to go against the grain, but I am unusual on this page. I am very close to my step kids. I am a mom although I am not the mom. My step kids have two moms. No BM didn't love it at the beginning and probably still hates it now, but I was in the kids lives since they were very, very young, and now that they're grown and gone, I'm still very much a mom to them. The trick is that you have to remember you share your role as much as she shares her. Your heart will get broken when the kids claim her as a preference. Her heart gets broken every time they claim you at all. 
If you would like them to take the doll that looks most like you, let one of them know it's your absolute favorite and you would love it if they would take her out on an adventure. If you want them to pick their favorite, isn't it lovely to know which ones they are drawn to and that they are your dolls. So even though the trip is on her time, you get to be part of the day. 

BM in my life would've either gone out and bought dolls, or made sure that they were not to bring mine (for the first few years where jealousy overrode the kids needs). Or she would have allowed them to get damaged (potentially) in her care. If these are truly treasures of yours be prepared that the kids may damage them. The hostility mellowed eventually, but when they were eight she would have undermined it so that my influence did not touch that day in any way..
Tread lightly so that you don't give up what seems to be a semi-cordial relationship, and remember that it will ebb and flow. Sometimes you're the favorite, sometimes she's a favorite. It's not about being the favorite. It's about the kids having lots of love around them.

edited to add: you want the kids to love their mom. If you want well-rounded, confident, healthy adults, the children should love all of their parents. So good for you for continuing to try to keep a good relationship between all of you

I agree with the sentiment that the other posters are giving you.  You are well meaning, but if you want to have the mom experience, you need to become a mother.. by having your own children.  These children may care for you, but they already have a mother.. and she is involved in their lives.  As civil and cooperative as she has seemed to be so far, she is not willing to step aside and give you "mom experiences"... the hosts of that party are inviting moms.. and you aren't the mom.  

It's nice that you are offering up some keepsake dolls of your own for them to use.. and don't discount or resent the good relationship you have with those kids.. they may love you, but ultimately.. your not their mother.. even though you may act in some typical roles.. you are supporting your husband in caring for his children. 

In the end, when chips are down.. your husband and his ex may tell you at some point your voice or opinion doesn't count because you are not the mother. not a parent.  I think you need to temper your expectations for being fully included as a primary member of that team.  You are a supporting role.. you are as others have said.. the friend/aunt/dad's wife.  you are cohead of the home.. but if they were at the top of a volcano with mom and you.. and they had to toss one of you in.. it would be you.. they will pick their bio parent. If anything were to happen to your relationship with their dad.. your connection would probably cease to exist.  It's painful, but it's reality.. and the sooner that you can figure out how to live with that acceptance.. I think you will find life easier and lesspainful

(though, when I used this analogy with my YSD.. she said she would push us both in.. haha)

You are in the early phase of the most difficult job there is.  Step Parent. 

It is good that there is a generally positive situation for you, your partner, the SD's and the XW.  However, be wary of the XW and overstepping.  It is close to that now with the XW wanting your AG dolls.  You already said the girls are not doll people but you are. Odds are that XW isn't a doll person either.  AG dolls are not just Toys-R-Us junk as you well know.  My spidey senses are buzzing that your dolls are likely not to return to you in the same condition that they leave in without your direct oversight of every second.  Go to the party.

So, IMHO, your stance needs to be your AG dolls go, you go. No need to stress the not invited crap.  If your SDs, DH, and BM are invited, you are a parent. You are invited.  Your DH is going, you are his mate, you go.  Not only that, the little girls and their moms who are AG ladies will totally welcome you. you are one of the AG true believers.  I would go and not feel a bit uncomfortatble about it If I were you.  If XW takes exception, then she will be showing you who she may really be under the facade of the co-parenting partner she is presenting  to be currently.

My SParent situation and experience is somewhat different.  I have no BKs.  DW has had full physical and legal custody since birth.  My DW and I met when our son was 15mos old and we married the week before he turned 2yo. In our marriage he is an only. In the toxic opposition Spermclan he is the eldest of 4 serial statutory rapist spawned all out fo wedlock half sibs (2) and full sibs (2) by 3 different baby mamas.   My DW and I are equity life partners and very early in our 30+ year marriage we set the fact that our marriage and each other are the only top priority.  SS was out top adult responsibility.  The SpermClan was not even a consideration nor was one bit of overstepinng, toxicity, manipulation, lies, or PAS tolerated.  THey played nice or they experienced escalating painful consequences that delivered to them a life of escalating abject misery.

Our son (32) asked me to adopt him when he was 22yo. He is a man of character, honor, and standing in his life, profession, and communtiy.  His  3 younger half sibs include Spermidiot spawn #2 who is on the dole, #3 who is in prison, and #4 who is not far behind the inmate.

Our situation was comparatively simple due to us never living nearer than 1200 miles to SpermLand and my DW having sole physical and legal custody. That limited the SpermClan to a long distance NCP visitation schedule of 7wks per year. 5wks summer, 1wk winter, 1wk spring. 

You have a local situation.  You did not mention a Custody/Visitation/Support CO or a visitation schedule. If your SO does not have a CO including a COd visitation and custody schedule, him getting one is a hill to die on .  It is critical that there is a CO in place as the foundation of a COD situation before anyone considers making a life with someone with failed family progeny and an X.

IMHO of course.

I was 4 years post divorce when DW and I married. Fortunately I escaped my blessedly short first marriage without sullying my gene pool with my serially adulterous XW.  DW was a single teen mom and never married prior to our nuptials.

You are already apparenty feeling like a marginalized add on. Do not let yourself become anything less than an equity life partner to your partner and a respected SParent to your SKids. Kids do not have a choice but to perform to behavioral standards, perfornance stnadards, and comply with the boundaries that you and your SO set for your home and blended family.  That means if BM cannot understand those boundaries and standards, she cannot be tolerated in your life beyond her COd time with your Skids.

As I indicated, SS is an only in our marriage. For various reasons.  If you decide to procreate with your partner absolutely do not do that or risk even the possibility of that without clear standards and requirements that he will be absoluted commited to you, your future child, protecting your family from any toxicity from his failed family progeny and his X, and being absolutely your equity life partner in life and the two of you being equity parents to any children in your marriage regardless of kid biology whether those are his or yours together.

Tolerate no bullshit and make absolutely sure that he is all in, your well being is fully protected, and ... read  and live the Step Parent Bill of Rights (SPBOR) below.

Good luck and take care of you.

Step-parent Bill of Rights

1-I will be part of the decision-making process in my marriage and family at all times.

2-People outside the immediate family - including ex-wives or husbands, in-laws and adult children - cannot make plans that affect my life without my consent.

3-I will not be responsible for the welfare of children for whom I can set no limits.

4-I must be consulted about which children will live with us, when they can visit and how long they will stay.

5-I will not be solely responsible for housework; chores will be distributed fairly.

6-I will be consulted regarding all family financial matters.

7-Others may not violate my private space at home, nor take or use my possessions without my permission.

8-I will never be treated as an "outsider" in my own home.

9-My husband or wife and stepchildren must treat me with respect.

10-Our marriage is our first priority, and we will address all issues together.