Dad should have yelled back "HELL NO!!! Lets eat cake!"   

And gotten eveyone else laughing about it.  If the two of you make a concerted effort to make BM and the SS's nauseating insistence on injecting her presence into every family event into an eye rolling laugh fest they will stop.

Public humiliation is an effective tool for modifying behavior. So roll it out.

Good luck.

"why would my husband's ex wife need a piece of his bday cake?"

So rude. Someone needs to teach SS24 some manners. Why does she need a piece of her ex husband's birthday cake?

They are divorced - deal.with.it.

especially after your organised and paid for the whole thing. Can’t forget about BM though can they?!! Not even for a minute.

I’m convinced they do it on purpose to p1ss the stepmother off. If I made a lasagne it was ‘mum makes an AHMAZING lasagne!’ and so on and so forth.

Horrible, nasty stepkids that are trying to rub your nose in it. Yuk.

The best revenge is to be happy. 

If BM is at an event, sit next to her. Talk about all the things your DH does that is cute, loving, affectionate etc. Then get up and give him a great big kiss. 

When skids bring her up such as the gift - get excited. Say oh I hope she loves hers as much as I loved mine. Give your DH a big bear hug and say oh DH having you is my gift every day.

As for the cake - I would have cut off a great big slice and passed it down. Then cut off an even bigger one while saying 'Is that enough or do you think your mom would like more? I know she likes her cake and to eat it to. Just making sure she gets a big enough piece'.

Skids love to get a rise. But when you give it to them the uncomfortable scrambling is great! 

SS24 is a man. Time for DH to pull him aside and ask him if he's trying to cause problems with his SM or if he's feeling like he has to take care of BM's feelings about the skids being with your family?  Either one is unhealthy and it's reasonable for a father to talk to his son about it. 

I would have said "Ohhhhh, good idea, lets cut a piece of cake for grandma too!" or aunt edna, uncle bob?  to take attention from BM

When is someone in dh's family going to get a back bone  and tell his x she is no longer a part of the family ie Thanksgiving for starters.

Secondly the reason adult kid is asking for his mommy to have a wittle piece of  birthday cake is because of my first question.

Is bm giong to DH50's proctology appointment too?

You sure do have your hands full OP.

Good for you and your conversation with your MIL ...  that was truly one for the books!!  You said all of the right things and it is absolutely true!!  Why would they want to carry on a relationship with someone who hurt their son?   I don't understand it under in any circumstances but especially in your situation.

My MIL does the same thing but she doesn't invite her to anything but carries on a friendship with someone we need to have police protection from.  I don't understand it.  I told my MIL it was the reason I could not be closer to her like she would want me to be.  MY MIL has repeated things to the ex...   However I also use my MIL now to repeat what I want her too.... Kind of fun for me !!! 

It is a very hard situation!  I wondered how your party went,  

I do believe that your Step-daughter was baiting you and trying to get a reaction from you.  When you didn't reciprocate I am sure she felt disappointment.  If wanting a piece of cake for her mom was what it was really all about, she would have asked quietly.  It was just her way of brining her mother's presence into the party and trying to make the in-laws feel bad because she wasn't invited.  

What normal person would want a piece of cake from a party for her ex-husband?? If she is craving cake so badly doesn't Pepperridge Farm make a cake she could pick up at the store ??? 

"Oh no, poor dear BM.  Here take her a couple of pieces.  Has the poor dear had any luck building a life yet?  I'll bet she regrets blowing up the one that she had."

"DH cut a big chunk of cake and passed it down..." 

The IL's and skids are being blamed for BM's constant presence but here is DH himself cutting off a chunk of his own birthday cake to give to the exwife.  If she is to be distanced DH has to be the one to set the boundaries with his family.  DH needs to be trained on how to react when dealing with these sorts of scenerios.  In my job we teach kids/adults with learning disabilities and autism how to deal with situations through "social stories". Perhaps DH could use some social stories when dealing with his family, sons and BM.  

Some examples could be:

When son brings up BM very publicly at the family gathering I will acknowledge him and tell him we will talk about it privately later.  

If son asks for cake for BM I will tell son that the cake is for the people at the party and leave it at that.  

If family wants to invite BM for the holidays I will ask them to visit with her at another time.  

I know it may seem silly to do this but I had to do something similiar with DH when he kept blabbing our personal business to his sister and sons and making promises to people that impacted our lives.  When the prying questions start DH now says, "I'm not sure, why don't you go ask still learning about that."  When people ask him to do stuff I have overheard him say the line that I taught him, "Let me run that by the wife first."  

He may actually appreciate a few simple tools and lines to say that will give him back his power.  

I have some advise. It is not good advise and I doubt that I really could do it but I would sure want to.

I would want to look stright at ss and say "Sure", then cut a piece of cake, spit on it, and then pass it down with a smile!

Staring at one another awkwardly, thats when i’d say “ummm why does your ex-wife need a slice of cake from her ex husbands birthday that she’s never been on good terms with?”

the idiotic smiles previously on sk face would dissapear real quick.

any birthday party for hubby i am catering for at home and baking his cake so no way in hell would i allow a sk who is a guest in our home to even feel she has a right to demand a slice of cake for mummy dearest. I’d actually ask in front of family why she even needs cake, this is her exhusband??

If you act like it bothers you they "win". I would have done the same, said nothing. Or told DH, "yes cut off a slice for "Jane", hope all is better with her". Subtle but pity because she is truly pathetic. Can you plan some birthdays and holidays away without his family? We had to do this to keep our sanity, but my DH was on board with that. 

The only one that should stop this bad behavior is the husband.  Period.  Tell him his encouragement of his kids and bm’s intrusiveness has to stop.  It’s all in his power, but I suspect he likes all the attention.