Do nothing to prepare for or facilitate this get-together.  Nothing. 

Even though these people/skids are "lovely" and don't appear to be mean to you, they are not there to see you - they are there to visit your DH.   You are exactly right when you say "them and me" and don't forget it.

If your DH tries to get you to help with the visit, tell him that you will leave all planning (meal prep, house arranging, etc.) to him since he knows them best. 

On Sunday morning get up very early and leave the house.  Go visit a friend, go for a walk, whatever.  Don't come back home until late.  On the way out, cheerfully tell your DH that you have other plans for the day and hope that his visit goes well.  

Then leave. 

He's "informed you" that his family are coming over - and what are you - chopped liver?  Do you not get consulted rather than "informed" after the invitation has been issued?   I would "inform him" that I had other plans on Sunday - "oh, didn't I tell you? well I'm telling you now!  Good luck with the cooking!"  

Hmmmm?  Nope, do not lift a finger or say a word to help with this gathering.  I would not leave the house either.

I would be obviously present and just as obviously not lifting a finger.  See how daddy does preparing the Luncheon for his brood without your coordination or participation.

Be relaxed and watch the trainwreck unfold.  If he asks you anything, answer with "Your event, you make it happen."  

Enjoy the disaster.

Oh - you just remembered that your good friend invited you out on Sunday! Have fun at the lunch, DH. (And have fun cleaning the house and buying the food).

What's with the BM being around???

Boundaries much???

Biteback, it IS hard to connect with people in a large group. I find that to be true even with my own family of siblings and their families. Since it sounds like you like the skids OK, I hope you can find opportunities in the future to connect with them 1:1. I know that's always my preference too.

Sounds like your DH was thinking ahead by arranging the restaurant.   My DH never thinks of such conveniences. 

I hate having my grown SD here to visit.  They live in neighboring Stares, therefore have to travel to visit. There are other family members here they also visit.  

After 35+ years of hosting and letting myself be used, I  no longer host or accommodate my adult stepkids.  My DH is disabled and they come to our home to visit.

We are able to go to a restaurant,  which I like doing.  DH nor his kids ever suggest it.  I feel comfortable on these mutual grounds; although none of it makes me uncomfortable now.  I am over it.

I set boundaries for myself.  I am civil, but do absolutely no hosting.  DH is on his own.  For six years I have not fixed a meal, offered a cold/hot drink, etc.  No invitations of any sorts from me.  It is up to DH to have them visit.  This works well for me.

I blame myself for putting every effort into making everyone but myself comfortable .  Long story, why I totally disengaged, but I should have never engaged; I should have just turned it all over to DH from day one, 40 years ago.

I know how you feel when you said you don't feel a part of that. There's no emotional bond. If not for your DH, you would have never met these people or be friends with them. I really feel left out when they all talk about when they were little kids and things that happened long before DH and I met. I have disengaged to a degree but I find it very hard to be rude. When these gatherings happen, I figure it's making my DH happy. I try to be polite, keep a lid on my anxiety and just like a root canal, think it'll be over soon. It's money in the bank relationship-wise because I may ask something in return.

For years I heard about everything in DH's past, good and bad.  SD purposely would bring up the good ole days.

Six years ago, I disengaged from DH and SD's relationship.  I now make it obvious I have had enough.  Depending on the situations how I do it, but I handle it immediately and cut it off.  It hasn't happened for quite some time now.

I don't care, any longer, if they leave me out; but at the same time, I don't have to be put in a position being forced to listen to it AGAIN.

I take it one event at a time.

I understand being left out. I hate when SS tells the grandchildren "Tell grandad what you got on your report card!" or "Ask grandad if you can have a snack first." and on and on. I don't think it's purposely leaving me out, it's just that I'm not the one they are there to visit and I'm not considered even a relative. The aunts and uncles by marriage are more considered relatives than the stepmom! It took fivev years before MIL stopped commenting on how I must be overwhelmed by her large family, like I was a guest and had no family of my own. 

I get lonely at these gatherings too, I miss my kids even more (who live at opposite ends of the country). The saving thing for me is the kids like me and don't realize any difference so they always hug me and talk to me just like grandad.

It's not so much that you have to endure the 'tales of yesteryear', it is the 'de ja vu' all over again and again and again that gets to you.  For YEARS I had to listen to the same tale over and over again about how DH caught toddler SD in his garden picking strawberries and eating them, "How adorable!"  "How cute!"  But as an adult her tastes changed significantly to the point of leaning over the kitchen island to snarl at me, "I HATE strawberry jam", as I was pulling my home-made strawberry jam out of the cupboard.  If I had to listen to the 'adorable' strawberry tale today I'd be tempted  to throw a freshly made jar at her not-so-adorable head.  LOL

DH and his kids do the "memory lane" game too. I think it's a pretty normal thing, but it's also a way to exclude you.

I had a major meltdown after an hours-long display of exclusion, where even SD's husband left the room and SS left the house. I was stuck. I even tried to participate at one point, knowing full well I wouldn't be welcome, but I had to try to change the dynamic or lose my mind. Turned out DH told me, essentially, that I couldn't join their game.

Eventually we left, meltdown ensues. Excuses, excuses, more excuses. Finally he got it through his thick head that I actually WAS being actively excluded, and he was one of the Mean Girls participating and encouraging that exclusion.

Hasn't happened since. Not that they don't do the Memory Lane thing at all, it's just reasonable and we move on.

Anyway, have you told your DH how you feel so excluded and invisible when this goes on? He needs to change the dynamic or limit the first family history wallow and demonstrate that he values you now as his partner.