Life is not all fair is it.. I mean.. your daughters have you full time and have your DH there full time (assuming he is a decent stepfather to them?).  It's sad that your girls don't have a father or other grandparents etc.. in their lives.. not just for material things.. but just because it should enrich their lives right?  but it's not fair.. and it's just life.. it's not anyone's job to 'make it up to them"..it is their life..

So as far as what's fair in gift giving.. I don't think your DH should necessarily be giving his kid less by trying to factor in the fact that his kid gets things from other parent or relatives.  YOu should each give to your bios according to your ability and desire..it might be equal between you and him.. or not.. it can depend.

BUT.. I think your DH should be pulling his kid up short when he lords it over your daughters that he has all these cool things.  

It's not nice behavior.

But.. in life, there will always be people that have nicer stuff than us.. they will have peers at school that get the nicer phone.. nicer clothes.. etc.. at some point they need to accept that SS may be spoiled by other people.. but that's not in their or your control.  I would actually teach them to be very gracious when he pops off with his "now I have two".. "ooooh SS.. you are so lucky.. we are so happy for you.. cool".   Perhaps when he sees it isn't getting under their skin he will find it's not as much fun to brag.

I don't think banning him from bringing things is necessarily needed.. but if he ends up with a new whatever.. your DH could also heavily encourage him to donate his older item.. like why do you need two laptops.. you are bringin them back and forth.. we will let a less fortunate child have your old one from here.

Why are both your daughter's without their dad's? You statement of them having different dads so they are with you 100% of the time makes no sense. They should both be spending time with their own fathers. Your husband shouldn't have to make up for where their dad's are failing. 

At the end of the day all the kids should be getting equal from your household. It doesn't matter why/how SS get's more with a bigger extended family. 

I have the same situation, DH has SD and I have 2 biodaughters with no bio father.

I just tell SD to keep her stuff at BM's house (BM instructs her to keep everything in pristine condition if she brings it ANYWAYS, so I just tell her to keep it safe, to keep it at her mom's). I get that it is hard, I feel the same way, I just tell DDs that toys, and stuff does not equal love. You have a lot of love and nothing can top that. Take them to do stuff (can be free like going to the park, go close to the airport and watch the planes take off, if you have a destination like the beach near by, take just them. If it's raining outside (but no lightning) go dance in the rain, have them help plant a garden (even if it has to be in little planters). make special memories with them, something you will not do with your stepson. do things that will make THEM feel special. That's what I do, and my kids LOVED it. They are 17 and 18 now and tell me that they cherish those memories

You have nothing to feel guilty about, you are providing for your children what you can afford and what is necessary.  What's sad is you are married to a man who does nothing to stop his child from being an entitled spoiled brat and instead encourages it by continuing to buy him duplicate items of things he already owns.

He will make comments like "sweet now I have two of these" in front of my kids.

Dad needs to shut this down. 'Yes son, you have two houses so you have duplicate items of some things so that you do not have to transport them. You do not need both items at this house. You need to leave the items your mom bought for her house there.'  Then Dad gets a box and packs up the item for its return on exchange day.

Just going to echo the others. The problem isn't that he gets more. His family situation is such that it's going to happen. His dad shouldn't have to cut down what he wants to give his son just because your kids don't have as many people to buy for them.

The problem here is your SS being bratty and bragging about all the stuff he gets. THAT behavior should make your DH think twice about buying his son so much. THAT should result in your DH delivering consequences and disciplining his son. My guess is that if SS bragged less that you'd feel less guilty.

We spent exactly the same on all 5 kids of our blended family and furthermore, each received the same number of gifts.  We did this in the interest of my stepparent religion, parity.  His kids also got gifts from their mom while my deadbeat ex did little.  On the other hand, my kids got gifts from gparents and extended family.  As others have said, life isn't fair.

But, nobody should be gloating and we would have jumped on that.  

I have three bios, the older two have a wealthy biodad, the youngest has an absent biodad.  I do try to give more to my youngest, but I'm pretty stealhy about it.

I 100% disagree that you and DH should give less to SS.  What SS gets outside of your home shouldn't have much to do with what he gets in your home relative to your DDs.  You need to be fair in your home.  SS shouldn't be penalized because he has more people to give him gifts, and it's not entirely about SS anyway.  Remember, your DH presumably gets joy from giving gifts to his son - why should that be taken from him because SS has other family who are providing gifts?

Now, fair does not necessarily mean equal.  We spend more on my two SDs than we do on our DD, even though the SDs get more gifts outside of our house.  That's because the SDs are 7 and 9, and toys, gifts and even clothes that they'll like are more expensive than the things DD will love.  DD has more packages to unwrap, but we spend more on each of the SDs, and that seems fair to us.

As others have said, the big problem here isn't that SS gets more gifts.  Your daughters (especially the older one) should realize by now that life isn't fair and there will always be people who have more or less than they do, in physical things and otherwise.  Your SS's attitude and behavior, and the apparent failure of your DH to notice and correct that behavior, is the issue.  If your DH struggles to recognize when his child is being a brat or gloating and lording his gifts over others, point it out to him and tell him that he needs to do something about it for SS's own good.  No one likes a kid (or the adult those kids grow into) who is entitled, ungrateful, boastful and unkind.  He does his son no favors by not coming down hard on the bad behavior.

The balance sheet really can only be addressed within the blended family household.  What one skid with a different set of extended family dynamics gets Vs what another skid gets with their different set of extended family dynamics cannot be mitigated. Nor should it be. It is not worth even trying beyond a teaching moment with your kids that life isn't fair and their StepSib has a different family than your kids have.

What can be fixed in the situation you outlined is to not allow your "spoiled" Skid to bring their extensive gift booty to your home.  That completely shifts the dynamic and does not allow that toxic spawn to ply their usual bratty bullshit targeting of your kids.  If it is not allowed to bring the gifts or to show off, all it is left with is lip service which your kids can easily be coached to address in real time when your Skid plies their usual crap.

KISS.  Take effective action. Do not over invest in things that you have no ability to control.