Your H needs to spell out to Gma in a neutral fashion why this won't work for your blended family and how it would only hurt the other children's feelings.

He can be grateful for the offer, and perhaps suggest some other way Gma can spend time with her GGkid. The key is to find a balance that allows for kids to spend time with their respective extended family, teaches all the kids that life isn't always fair, and avoids situations that allow for egregious favoritism. 

Is there an age difference between your SS and your other two children? Maybe Grandma wants to take the two children who are the same age? Maybe she doesn't want to take 4 children to an amusement park by herself?

It sounds like there is history here that make explain our frustration - but without details it is hard to know if this is really out of line or not.

Do you think that grandma should take your two bios that are not with her son on the trip as well? or are you just upset that she isn't taking the other two that ARE her grandchildren?

Honestly, I'm not sure about the age of grandma.. but taking TWO kids is probably about all she would want to do.. financially and logistically.  I mean.. grandma taking 6 kids by herself?  (how old are her other two grandkids btw?).  That would be just about negligent to allow one person to take 6 young kids (3 at least are 10 or under) to an amusement park.

Do you think that perhaps the younger kids aren't going for that reason? she doesn't feel she can take that many kids and the younger ones will get their "turn" when they are older?

Guess now it's time for me to start being a b and not allow any of my husbands side to see any of my children. 

This would be a very bad move and seen as mean-spirited on your part.  The fact is that things are not always going to be equal with your kids because as a brood they literally have three sets of parent/grandparents.  If you really want all the kids to go to the amusement park why not offer to go with grandma rather than expect her to take them all.  Where are your parents in all of this and why aren't they taking all the kids out?  

Drop the control and facade of fairness. It's not fair, but neither is it fair that between you and DH there is a labrinth of family entanglements for the kids to wade through.  The kids need to be taught point blank that because they are in a blended family things won't always be equal but that it's okay. SS has been on the scene longest with gma so yes there likely is some favortism, it's human nature.  If grandma wants to take her older bio grandson out for the day with his cousin, let her.  Why should she have to take two unrelated kids or a 4 yr old who would want to ride the kiddie rides and not be able to do what his older brother does?  

In my situation it's obvious that DH favors SS's youngest child over the oldest who is a step.  He doesn't say it but I can see it. And why wouldn't he? This is his only grandson. The parents pawn both kids off on DH at times and DH doesn't complain but I know he'd prefer to see his grand alone at times.  

Your older two are being excluded from some things for a couple of reasons.. first.. while GGM is nice enough to "pretend" that your kids are her greatgrandkids.. in reality.. she doesn't see them as the same.  Part of the reason is that you do make it hard on her.. not letting your kids do certain things etc... She is also an old lady..now.. if she, her daughter AND your dh all had their kids at 15... then the youngest she would be is 45.. but she is likely at least 10 years older.  See how YOU feel at 55 years old and whether you want to be responsible for 4 kids.. (much less SIX.. if your youngers went too). I doubt it.

Your younger kids are not really ready for a trip like this with the older kids.. they will have different interests.. need a LOT more care and maintenance.  And.. you know what? it may not end up being equal down the road.. Great Granny may be beyond the age of being able to do these things with your younger kids.. it happens a LOT.. My DHs brother's kids got a lot more from their grandparents than my DH's kids.. they came along first so that's just how it worked out.

You need to learn to control what you can control.  you cannot MAKE your DH's grandmother take your older bios.. she has no obligation.. and again.. probably has been playing "lip service" to them being her great grandkids in an attempt to be nice to you.  Do NOT make it a big deal or you are likely to find that there is more fallout with your husband's family.

So.. what you can do?  You can take YOUR kids on a fun outing the day that SS goes.. so your kids have something too.  You can make your DH be a parent and not let his child rub the other kid's nose in it.  If he can't manage that.. when SS starts on his bragging.. tell the kids.. "hey.. let's hop in the car and go get icecream".. when SS10 starts to go.. you say.. "no.. you clearly told us you had a treat the others didn't have.. so you can sit here and think about how it feels when others get things you don't".

Again, you can't expect equal treatment in a situation that isn't equal.. neither in biology.. age etc...   All you can do is help your children understand that sometimes others get things.. and they won't.. but vice versa too.  And.. maybe you could also ask Great Grandma if it would be ok if YOU joined them that day.. and brought the other kids.. that way your kids get to go too?

Your expectation that your in-laws will treat your non-related kids the same as SS is unreasonable.  Your older kids are not their grandkids; they owe them nothing but to be kind, IMO.  Now, it would be nice if they included them - my parents have taken the skids to Disney, to the beach, bought them very nice gifts, etc. - but it is not something that should be expected.  If your SS is bragging about things he does with your H's family, then your H needs to discipline him.  That is not acceptable behavior and should be nipped in the bud.

As for your younger kids, time will tell.  It may be that they're excluded because of age, or it may be that they're excluded because of favoritism.  If the latter, and it's obvious, your H might want to say something about it. If he doesn't, or if it continues, then I'd be sure to do special things with my kids to try to make up for it.  And of course the relationships will be affected.  I vividly remember my grandmother favoring my cousins when I was a child, and I never developed a close relationship with her.  I never felt the need to call or visit her when I got older, either.  Her loss.

I’d leave whether SS goes up to DH (it would be a big fat no for my DH), and I’d just cut off his parents as much as possible, which is what I’ve done with my MIL.  She gets one token visit per year, just because she birthed my DH, and I’ve determined she’s insane more than she is malicious.  She actually had the audacity to ask for all the kids on their own this year, she can’t treat them fairly when we are there watching, freaks out if we say anything, what makes her think we’d leave them alone with her?  She’s a nut job.

I misread that this is DHs grandma, so you shouldn’t have to deal with her much, you can’t control her or your DH or your SS.  If you make a big deal about SS rubbing it in, it becomes a big deal, teach your kids to be happy for others good fortune, and it will take the wind out of his sails.