I was a little apalled when I first saw his girls at the family Christmas when they were 5 and 9.  It was at the grandparents and ther were 5 grandkids (including my SD's)... There was a MOUNTAIN of gifts under the tree... Granny had made sure the same amount was spent equally on each child.. and it was several hundred per child.. 

As soon as they got the word go.. the kids attacked the pile.. ripping and throwing paper behind them.. barely acknowledging the actual gift they had in front of them.. and at the end.. when you could hardly see them for the pile they had unwrapped.. the 9 yo looked up and said.. "is this all there is?"

I was floored.  They just experienced an amount of gifts that was unheard of in my house growing up.. and they wondered if there were MORE? when they hadn't even hardly noticed what they were already given?

Later, I talked to my DH about  how it really wasn't helping them to allow them to not show gratitude.. and appreciation.. for what they did get.  Future Christmases were different... the grandparents adopted a mode where people would individually pass out and open gifts so everyone could appreciate what was given and recieved...  ultimately.. the kids actually found they liked that better..it lasted longer and they got to enjoy what they were given.. and enjoy seeing others open what they gave to them.

You can't control granny.. and you will be seen as the grinch if you try to take their gifts... but maybe your DH could steer his folks into investing in a college fund with some of the money.. and more target a few nice things the kids really want vs just a mountain of crap?

My brother has this rule for grandparents (at least MY parents) where he wants them to contribute to a college fund. 

I think your husband should talk to his mother about this, maybe take photos of the things that are donated that are from her...

It certainly does happen in a lot of step households.  The SKIDs get spoiled silly to try and make up for the fact that their parents are not together any longer. The amount of money wasted on my two SDs used to drive me round the bend.  It was just so different from the way I brought my own two daughters up.  Sigh. 

SAme thing happens and my SD is also really into to getting presentsbut then doesn't do anything with them once she has them. 

Every time we go out she has to make DH buy her something. Every time the in-laws see her they come bearing gifts, every time. It's the same with them if we're out she will always try to get them to buy something for her as well. whatever she wants is always something that she is so interested in and it's her new obsession and she must have it and then it sits in her room untouched and unopened. I swear she just wants people to spend money on her to prove that they love her or something. 

When she opens presents she acts very melodramatic. It's like she's accepting an award on TV it's very strange and over the top. I guess it's good that she is thanking people for the gifts but it just comes across as so fake and weird and then she turns around to completely ignore the gifts unless they are clothes with giant brand names on them.

Slightly different family configuration than you describe, but my YSS's ex-brother and sister-in-law are fabulously wealthy.  Sister-in-law is a twin with his ex-wife and has no children of her own so she lavishes everything imaginable on his 3 girls.  It has created problems but thats a different story.

They would bring the kids to our town for Christmas and both we and BM would do the Christmas thing.  Later in the year, when we visited, ex DIL showed me the closet where she stored the excess.  Plus, each child's closet and room were stuffed with possessions.  It was crazy.  We opened college funds and only gave a token gift for each to open.

I had to stop my mother. She was the #1 spoiler!! She is still over the top but she has toned WAAAYYYY down. For the last 7-8 years I have told her that she can only do experiences with the kids. Get tickets to a play, spend the night out of town doing something, TAKE them shopping and make a day out of it. No more just endless gifts for the sake of it.

Both our household and my ex's have toned things way down too. The entitlement was just getting stupid. We try to stick to the 4 gift rule: Something you want, something you need, something to wear and something to read. Then we also try to unveil a family trip for the spring. 

It was a pretty brutal  fight but I told my mom that she doesn't have a choice I will return gifts. 

(All of this got thrown out the window as soon as she was diagnosed with Cancer last year. I will have to break it down for her again after she is done with treatments. Christmas and birthday's this year were stupid... but you can't tell at your mom with cancer LOL)

You are perceptive to understand that the excessive gifts can undermine the parents.  I wrote about YSS above, the one with the wealthy inlaws.  I dont know the entire story but I know the SIL tried, and was often successful, in exerting influence in their marriage.  I think it was one of the factors in the split.

My SGD29 has a similar thing as you with the excessive gifts from her inlaws.  She just had a baby girl in August.  She showed me the baby's room.  The closet is jammed, wall-to-wall, with baby outfits.  The child could never wear them all before she outgrows them.  And, we all know that babies are seldom in "outfits", mostly just in their jammies. She showed me the dresser drawers, all jammed with baby stuff.  She didn't know what to do.  I told her people aren't really buying for the child.  They are buying because the stuff is so cute, also perhaps reliving what they wish they could have afforded for their own.  In other words, they are buying because it's fun for them.

Luckily, DH isn't into the gift thing. SS10 gets a reasonable number of gifts from us at Christmas and birthday. That's it. DH's mother spends about $150 on him (she's wealthy with one child and grandchild but she doesn't think a lot of material possessions are good for him).

Our Problem comes with BM and her parents. They spend at last a couple of thousand on him each, per event. Then he'll get $100 just for a "pre-Christmas gift." BM will buy him whatever he wants throughout the year (like a new Nintendo Switch when he already has one that works fine). For a while, he got a $50 every week he didn't get in trouble at school. DH refused to do that. Behaving in school is just what you're supposed to do. You don't get rewards every week for doing what you schools just do.

It's kind of sad because he shows little if any interest in Christmas or birthdays. Why should he? He gets what he wants all year.

It drives DH crazy but he has no control over what BM and  her parents do. He's already showing major signs of being spoiled and entitled. But at least he knows better than to ask us for stuff. When he used to, he was told he could use money from his piggy bank or save up. So, he just waited to go back to his mom's and had it by the next week with us.

He's only 10 so the results aren't obvious yet, but what I see blows my mind. My sisters and I usually got whatever we wanted for Christmas and birthdays. Sometimes we even got special gifts. But we didn't have gifts lavished on us. But then, we were selective in what we asked for. Those gifts are great memories. SS's gifts here, he'll show vague interest for a day, maybe, then they sit in his room untouched until they're donated.