I read your background info and could relate.  We also had my 2 and DH's 3 full time with the dysfunctional bio dad and bio mom in the background.  Our 5 kids all got along, well enough, too.  Like your family, neither bio parent saw the kids very often.   It surely is a challenge and any holiday made it worse.

Hang in there.  December 26 will be here soon.

BM and GM Have no say in what's goes on in your home.  You dictate what comes into the house.  And what kid gets what.  I know you are trying to be fair. So,,So, tell BM and GM  That if they buy for one they buy for all.  Like I would not allow GM to buy only her bio cell phones and not your kids phones.  If she wants to get phones she follows your cell phone policy .  And she buys ohones a proper time or phones get disabled 

Good for you for enforcing standards in your home.

We purged video games from our home when SS-31 was in 6th grade.  He was obsessed and real life suffered, so... no more video games. The SpermClan was hell bent on gifting him all things video games.  It started with full blow video game systems... Nope. The went immediately into a locked storage room in our home.  Then they started gifting him the latest and greatest hand held gaming systems. Same thing. They went straight from his luggage to locked up upon his arrival from SpermLand visitation.

On the keeping things equal note.... nope.  Life is not equal, nor is is fair.  We never gave a single thought to his three younger half sibs by two other baby mamas in SpermLand.  Their fee fees, or the fee fees of anyone in the SpermClan were not even a remote consideration to us.

SS had nice things.  That pissed them all off.  Their response was to take his nice things while he was in SpermLand for visitation and return cheap flea market clothing, shoes, etc.... SS's nice stuff would show up in SpermClan family photos on the three younger half sibs and when SS was adult sized in his teens, his nice clothing would show up on the Spermidiot in FB posts.

We have our attorney send them threats of charges and law suits.  Suddenly, SS's stuff would show up.  Of course we were mean, and evil. No recognition from them that being thieving POS people is what is evil.

So, I would be wary of the whole make it equal for StepSibs on both ends of the blended family map.  Make what you do for all of them balanced, what the other side of the equation does, I would be hesitant to balance that out.

Rather than force BM and GM to give them all the same thing, considering that your BKs have zero standing for BM and GM, this is a teaching moment for your kids to learn reality.  On that note, if you want to make your home a no kids with cell phone home, enforce that standard.  No cell phones. If the Skids show up with phones from BM/GM, they get locked up until the SKids are on their way back to BM's for visitation.  But, no cell phones means no cell phones.  Yours can't have them .... ever.... if the Skids can't have them in  your home.  

An X has zero requirement to recognize Skids in their failed family breeding partner's life.  None whatsoever.  Even if their failed family breeding partner has young children with someone else and are the failed family progeny's half sib.... the X has no duty to recognize or gift that child.  If they are decent, they should be decent about their kid's half sib(s) but there is zero duty to buy the half sibs gifts.

Now, if the GM in question is your MIL, that is another thing entirely.  If that is the case, you and your SO need to put your collective foot up her ass for that crap.

Be careful about trying to force your SO's X and XMIL to provide for  your kids who have zero standing in that X's and XMIL's life.  Blended family life is complex enough without turining a nothing into a something.

IMHO of course.

My SS is my parent's eldest GK.  We have never used the "Step" label in our family.  He is their eldest GK but not their first GK.  DW and I married the week before SS turned 2yo. My niece was 5mo old when we married.  My rents have never treated SS as anything other than their GK.  Mom and dad are extremely close to my DW and to my SS. He asked me to adopt him when he was 22yo. We made that happen.  My mom, DW, and SS were all together shortly after SS told me he wanted me to adopt him.  I was overseas.  There is a great  story about them being at lunch when DW engaged our son with Isn't there something you want to share with your GM?"  SS told mom that I would be adopting him...... then..... "Mom, Gramma is crying, what do I do?".  "Go hug your grandmother son."