This seems less like a step family issue and more a "people are a-holes" issue. There are simple solutions here that would allow a party (if you want one) while keeping the a-holes at bay.

First, your DH could temporarily block BM's number for the day, or keep his phone off. I doubt anything is *that* big of an emergency that it can't be handled until after the party.

Second, don't invite the in-laws. If your DH wants them involved, then he can throw a separate party with them. They have proven they can't be civil at a party and are therefore no longer invited. I'd advise the same if it were your parents or friends mucking up the event.

My SIL runs into this with her family. Her mother has trotted men in and out of her life, which has created loyalty binds for my SIL. Quick backstory: my SIL's Dad is not her bio-dad, but he raised her. Then, she had a SF up until a few years ago (when she was in her 20s) that she cared about. Now her BM is remarried, and while SIL doesn't see him as a father-figure, she respects him as her mother's husband. Add in that she just connected with her BD and her Dad just died and it's all a hot mess.

SIL has had to tell the adults to grow up and be polite, and if they can't, they just won't be involved. SIL's BM gets mad because Niece spends more time with my SF and Mom than her. SIL told her that her creating drama with BD, Dad, SF, and New DH has made her consider how much involvement Niece has with her. Her BM would screech when SF and Dad would be at a thing because she found it disrespectful to her, yet never considered that these men helped raise SIL and SIL loves them (they did nothing wrong other than marry crazy).

So, it's totally possible, and perfectly acceptable, to tell people that they can follow your rules in your home or not be involved.

Holidays and birthdays were a never ending source of stress while the skids lived with us.  Even now thatt they no longer live here, they are still drained of all joy because it still manages to be all about them.  I have no clue how to change it given that any "family" celebration includes people who bring drama and chaos by their very presence.  I started avoiding them as much as possible and not reminding DH so that he would be busy scrambling to cobble something together at the last minute.  For your own kids, there is no reason that you can not take them on a special outing or something with their little friends from day care or pre-school on a day that just happens to be one where the skids are not around.  There is no rule that you must do a cake/grandparents/presents/stress party.

I get it. Every "special day" some how always turned into a "skid thing". All birthdays (even if they weren't theirs, father's/mother's day, 4th of July, Christmas, even just casual "family bbq"...whatever). Almost like it was "skids are the most important persons on the planet day, let's all rejoyce at their awesomeness day". 

Both my formerSO and formerMIL always had to make sure to highlight skids and almost...idk, discount my babyBS to ensure skids didn't feel "replaced". Like they thought the skids should be some how highlight and and overcompensation to ensure a spotlight was on them because they were "poor COD". It drove me freaking insane. 

Well, I guess that's a positive side effect of alienation.  BM makes sure DH never gets to see SS on any special occasion, even now that he's 19.  So, no drama.

 I am known in my circle for hosting pretty good parties but because of what happened at my husband's 50th birthday party 3 years ago, I have hung up my party planning hat until one of my own children get married....

  It was an event 6 months in the making. I planned it no differently than Napoleon would have, going into battle down the the smallest detail.

  It was my intention to have an adults only affair since we were having an open bar and didn't want to take the responsibility of minors drinking at a hall rented in my name.  As it was only my time and money invested, I had no idea that my husband had invited his 16 year old daughter and her friend until they walked into the party. It was her mother's week to keep her but heaven forbid we have any sort of fun without her front and center reporting back to her mother in real time.

 I'm a little bit of an amateur photographer and had taken so many wonderful pictures of my husband from the time that we had met as well as some old photos that his father had given me from his childhood. I made a huge photo collage and put it on an easel in the corner for the guests to see. There were a few pictures of my husband and his children included but apparently the stepdaughter felt that it should have had more pictures of her.

   She literally sat at a corner table and cried her eyes out with people coming up to her to ask if everything was ok meanwhile throwing me under the bus and telling the relatives that were in attendance how much I hate her because I put more pictures of him and myself on the easel even though her and her brother were in his life much longer.  My husband interrupted my serving food, cleaning tables and emptying trash to let me know that it was MY fault that the special snowflake stepdaughter was crying because I didn't even say hello to her but she walked past me numerous times while I was setting up the buffet by myself and didn't say a word to me or offer to help. I reminded him that I had planned an adults only affair because I didn't want to have to deal with her drama because I had a lot of work to do with absolutely no help from him. He went back to his daughter and guests with an attitude and literally did not lift a finger to assist me before during or after in any way but had the audacity to blame me for hurting my 16 year old stepdaughters feelings.

Never again.

I turn 50 next year and have already started planning a cruise. With or without him.

I claimed a conversion to the Jehovah's Witnesses who specifically do not celebrate holidays or birthdays.  Even got some literature and left it scatterred around the house.  I thought it was a stroke of genius but my DH for some reason was not amused.  Weird, huh? 

Why do you feel obligated to host a large, adult-filled birthday party for such young children? They won't remember it, and would be just as happy with cake, ice cream, and a few presents after dinner.

Just be done with the madness. In a few short years, your bios will have little friends and you can have them over to celebrate, or parties at trampoline parks, Chuck.E.Cheese, or whatever. Make birthdays about the child and not a quasi family reunion. Have those at a local park (so you can leave when things get ugly), or have just a few people over at a time.

OK for fun and since your husband is till connected to the cord, HUGE RED FLAG IF YOU DON'T ALREADY KNOW...

Here is what i would do. Reserve the party location where hubby wants...not at your house becaues YOU cant leave, your held hostage sorta.  Tell hubby to invite ALL the relatives he wants to. YOU will call the joint and reserve with deposit on his card Then the night before party YOU  get the sniffles. Party morning tell him you feel "awful"  and can not possibly gooooooooooooooo...your so sorry. You hate to miss all the fun. LOL*shok*

Its not a lie because you have had the sniffles from all the crying you have done,,right? You do  feel AWFUL. That is not a lie either because you do feel awful for all the shit going on.

Husband can handel the entire party with the joy of having allllllllllllllll his family with him. That is what HE wants, right?  You can enjoy a few hours at home and on your own terms. Have a little tiny bday cake for your daughter, go get some balloons at the dollar store.   Its amazing how much  better you feel. OH dont forget to tell husband...no company allowed after the party since your under the weather.

Listen your daughter will not remember this year or maybe even next year. It may be familiar to her if you show photos. It is the photo with familiar people that she will remember not necessary the event itself..

There is more than one way to skin a cat.

Lesson here OP is, control your own holidays. IF dh wants his parents involved---he can, Have the worlds biggest jumpyhouse on earth at their place. Have the party at INLAWS place. Reserve a small room at local pizza joint, DH pays and throw a party.

Start taking control of your life, even IF in small ways. Its freeing.

this 1 day to celebrate your childs birthday is turned into revolving about the stepfamily and their issues. Things that aren’t urgent and can be discussed after the party or next day are brought up as immediate issues. Typical stepfamily “i’m the centre of everything mentality”, they need attention on them always.

i had issues with my husbands own family members coming over to our daughters birthday party which i prepped and catered for personally, sick and contagious, we didn’t know because they weren’t showing symptoms but they were dumb enough to leave their meds at our home, hug my newborn son. 

People who come to your home with no manners or respect don’t deserve to be invited. Screw them sulking saying its wrong for you to not invite them that this is family etc.

i get you feel guilty because they’ve put you in this place but will accept no fault for it. The past 2 yrs we did our own party at home where i baked or bought a birthday cake and did roast chicken for dinner. Its an intimate party but you know what, your kids are happy seeing the attention on them for a bit and blowing out birthday candles etc.

when they’re older and want friends from school then do that. If the inlaws get all shitty saying how dare you not invite them, hubby needs to tell them that they haven’t been decent at previous celebrations with their drama and ultimately this celebration is for your bio kids, they are just stepfamily members so its not a priority they be present

I totally get this. 

BM isn't a problem for me, and neither is DH's family, but SD7 herself can be... She's become pretty self-centered and doesn't often appreciate effort put into things. For example.. Last year for valentines day, I spent over an hour putting together sweet little treat bags for her to hand out to classmates. I also picked up a few cute little things to surprise her with in the morning. Naturally, she didn't thank me for ANYTHING I did. When she got home from school, I prompted and asked if she liked everything/if her friends liked their treat bags and she just said YUP.

She proceeded to sit down and begin eating all her candy and said "I'm going to save some for daddy when he gets home" and low and behold, when dad got home, she ran over to him offering him all sorts of things. Did she ever think to offer me something? Nope. Me - the one who planned EVERYTHING for valentines day for her. So this year, I'm not getting her CRAP. Nothing at all. Which of course, means I'm not getting my son anything because it would be "unfair" 

Ugh. 

The only reason I don't want to celebrate Christmas is because my FIL is a miserable pain in the arse.  Even when MIL was alive, they'd both sit there and moan about people until I was near tears.  Ya know, cuz it was Christmas Eve, we were having a meal to CELEBRATE.  NOT b*tch and moan about everyone and all their faults.

Now FIL is even more miserable because he just wants to die to be with his wife.  So he's twice as miserable as what he used to be.  So Christmas has become a disappointing sh*t show.  FML.