You should not have to take step grandkids on this special vacay.  Especially if you barely know them.

The liability alone would be enough for me.  You aren't related!

Well.. you know you DON'T have to take the step grandkids with you... of course not.  YOu don't even need to take all your bio grandkids if you didn't want to.  It's your gift to do with as you please.

however... and I'm not saying this because I think you should take them but... you said the kids didn't give it one thought... you are getting this question from your son... maybe he DID think it was unfair just never said it as a child?

It's crazy that your son thinks you would be ok supervising that many young kids though.. THAT is surprising to me.

If they feel all that badly.. give them the information for the cruise and let them book another cabin so his wife and sd's can come along (their expense)

You do not need to take the step-grandkids on vacation.  If your son is not willing to let his daughters go with you unless his step-kids go as well, that's his choice.  But he cannot tell you that you need to take unrelated kids on vacation, and for him to do so was presumptuous and unfair.

FWIW, my parents occasionally take my skids on vacation (with us).  The last trip, however, was just with our bio - their biological grandchild.  It is 100% their choice, and I would never demand that they take the skids anywhere, babysit the skids, give the skids gifts, etc.  It's their choice, and I fully expect that our bio will go on more vacations with, spend more time with, and get more gifts from my parents than the skids do.  On the other hand, the skids have real grandparents who can do things for them, so it's not like they're deprived.

Your money, you invite who you wish.  If your DS pulls your GKs because he wants his SKs included, take some friends and let your DS know that his perspective is illogical.

That said, my SS-27 is my parent's eldest GK.  His mom and I marred when my niece was 5mos old and 8mos after I introduced my DW and SS to my parents.  My parents would not have a GK trip without my SS being invited.

But if they did, that would be their choice.

I think it is fine to take just your grandkids. I didn't always feel that way....at least not at the start of this whole blended family thing. I was blind to think that my kids would be welcome as "grandkids" to DH's parents....they were not. I was blind to think that my family would welcome step kids as "grandkids".....they did not. It's hard as parents of a blended family to balance and juggle it all....when we really want to be one family.....but I have learned that is not in any way possible.

Now....what my Step mother in law did takes the cake......not only did she take just my skids and not my kids...keep in mind these are her step grandkids... (and 2 of her other "real" grandkids")  on vacation...but she took BM!!!

Step MIL invited her "real" grandkids and their mother..........her step grandkids (my skids) and BM.........yes you read that right. DH was married to me...and his step mom invites his ex wife to vacation with her and his kids........the relationship between my DH and step MIL will never be fixed. I hate her guts because she pulled this sort of stuff with BM and skids for the first 2 years of our marriage and is now trying to make it right.......too late. We just act like she doesn't exsist.

But in your case...........yes take only your grandkids.

First you have no legal right to SGK.  If something happens you can not make any medical decisions for them. You have no rights to control SGK.  If you touch them that can be assault.  If they make up something about you, you will be in deep   sh*t.    
your som may not let you take GK by themselves,  that will be there lost. 

You have no obligation to step-grandkids. My parents were kind to my SS and bought him a small gift if he was around at Christmas, but that was it. They would have never dreamed of taking him anywhere, and I would never have demanded it. He has two sets of grandparents and my parents aren't one of them. 

It's your DIL.  I am quite certain that your DIL does not like seeing her own kids treated differently and had your son intervene.  She was wrong.  And your son was wrong to do so especially if you already include skids otherwise. 

I don't think there is anything wrong with kids having a special relationship and/or special time with bio grandparents.  It is your DILs responsibility to explain, kindly, to her kids that they may not be invited to every special activity but that doesn't mean THEY aren't special to GrandmaShill.  She can tell them in an age appropriate way, as an example:

"Susie, Timmy - you know that sometimes I like to take each of you on special activities so you can have 'mommy' time alone with me, right?  Susie, I take you to the garden club with me and Timmy, I take you to the library.  I know you both like these special times we share, right?  Well, GrandmaShill likes to do the same thing with Bobby and Mary sometimes.  This is her special time to spend with them.  It doesn't mean that you two aren't special to her too, it just means that she wants to have some special memories with just Bobby and Mary."

Of course, getting your son and DIL to explain this is something beyond your control.  So is your son allowing your grandkids to participate.  If he declines, then there is nothing you can do other than tell your son that you are deeply hurt but will respect his wishes if he denies his kids this opportunity to make memories with HIS mother. 

It will be on him to live with regret, if he has any.  

Welcome your son to spend his own money and to come along.

I would say take them IF you could afford an extra room and IF you were not by yourself. Four kids on vacation with grandparents sounds exhausting, my parents wouldn't be able to handle it, nor would it be safe.

Besides....four kids are expensive. When you have a big family with a lot of kids, not everyone is going to be perfectly equal all the time, especially if you aren't wealthy!

Maybe at a later time, you can spend some special one on one time with your step grand children. Take them to the children's museum or something.

I have taken all four - 2 steps, 2 naturals g'kids - on a few trips but I've taken more trips with just the 2 natural g'kids.  And I'm always the bad grandmother on those trips where it's just the two.  

My two step grandkids have three sets of other grandparents (due to remarriages) that take them on trips and buy things for them, without including my two biological g'kids (one of whom shares the same mother).  We've tried to explain that they will have some separate trips and some together trips, just due the family structure with the ex's families.  Is that an option for you?

My attempt to improve the situtation was to take the trips with the two natural g'kids on the dates when the step grandkids are with their other family.  The kids seem to notice less when it's out of sight and they are doing something fun themselves.

Another solution I've offered was to take the stepkids on their own trip with me for their special time.  But that would double up on trip costs, which may not be an option for you.

I do agree with others that trying to juggle 4 by yourself is a handful and may end up making you too exhausted to enjoy the trip.