you don't handlle it.. but your DH should.  he should tell his  mother that it is unkind to not show equitable treatment for all her grandkids.  ie photos.

but.. yeah.. my MIL treats my BIL's family as golden.. my DH's girls are great but she is hyper critical of them.. the irony is if it weren't for my YSD.. she would be dead since she wsa the only one to give a damn enough to take her to the ER when she almost died of pnemonia a couple of years ago.

What would happen if you pointed out the obvious in a joking way? "Jeeze looks like we know who the favorite is based on all of these pics!" Say it in front of a crowd to public plant the seed... Make it light hearted a little "I was just kidding around" you'll probably strike a nerve with her- but calling attention to it might force her hand to make her change? If you notice- someday your kids will, too.

maybe stop taking them over if it's so glaringly obvious that grandma favors one child over the rest. say something like "it's hurtful to favor one child and since it isn't going to change, I'm going to protect our kids and give them attention from me. I don't want them to be hurt when they figure out how grandma really feels but you enjoy the time w/ SD and your mom. I understand you are fine w/ it but I am not."

Playing favorites is all-too-common even in so-called intact families.  My paternal grandmother favored my uncle  (the younger of her two sons), his wife and  children over my mom and us and made it painfully clear.  She treated us like servants whereas she was always treating my cousins out to various places like Marineland and Game Farm.  Even when we were caring for her in her old age and practically on her deathbed she did nothing but bragged on my uncle aunt and cousins.  My mother finally spoke up and said "where are they now?  They certainly aren't taking care of you but we are!"

And not surprisingly my mother favored my younger sister over me with boatloads of photos and practically no photos of yours truly. 

Even my kids noticed it.  Fortunately my sister is childless so the opportunity to favor any children of hers has been nixed but I'm sure it would have occurred had she procreated.

I would just let it go. Same thing happened with our family. SD is golden child with framed photos everywhere. My two boys could care less. MIL knows what she is doing, that is her choice. There are consequences. 

I deal with a very entitled SS14. On DD2's first birthday my MIL insisted on making her cake (she's a baker). Anyway, SS INSISTED that she make him a cake, too. I shut that shit down quick... publically... so that he could learn that not every day is a celebration of him- that he can be happy for someone else and enjoy a piece of his sister's cake. He was so pissed he skipped the party and went back to BM's house two days early. Too bad, kid! MIL respected my boundary and did not make a cake- but she did drop off a piece of cake to BM's house the next day so he could have a piece... I was fine with that (that's entirely on her and she respected my boundary) but I would be dammed if she made him a cake on DD's day. 

The stench of the blatant favoritism is something that they all three should have their noses rubbed in regularly.

My in laws, favor SD14. She acts like she can't stand it but hams it up to get whatever she can out of them. They wait on her hand and foot and always have a gift for her no matter who's birthday it is. It seems the only time my in laws see SD if for a birthday (BM won't give up holidays) and she always gets a load of stuff bigger than the person of honor. 

My MIL not only plays favorites, but also tells SSs that DH gives too much attention to DD and not enough to them. She also told them that their inheritance "will go down" now that we have DD (not sure what inheritance she's talking about, DH had nothing but debt when I met him). 

She also plays favorites with BIL's kids. For her birthday, MIL said she was going to have burgers and fries. DD was 10 months old, but I figured she could eat part of a burger and some fries. Then SIL wasn't attending, so MIL served leftover bacon for BLTs (10 pieces of bacon and 1 tomato for 10 people). She made a special pizza for DH's 9 year old niece, but had nothing that DD could eat and didn't tell us in advance, so I hadn't brought anything for her. I had to cut up fruit and give DD some cheese from the charcuterie tray. We also have to bring everything to MIL's house (high chair, toys, place to sleep) every time we go with DD. Meanwhile, in pictures when SSs were young she had a swing and a high chair. She also had an entire bucket of toys for BIL's kids when I first met DH. She'll say, "we're just not used to having a baby", but DD is now 18 months old, so she's had some time to figure it out. 

She's done the same things with photos, DD's photo is hidden in the living room where no one goes, but there are pics of all the other grandkids in the main sitting room. She then told me she just needed to get a new frame for DD's photo...meanwhile, the ugly pics of SSs from BM were prominently displayed immediately.