Is it wrong for me to have told my fiance that he can get the gifts for his kids? And I will take care of mine? I still handled the family Santa gift and stockings. 

That is perfectly acceptable and is really how it should be done.

Is it wrong for me to tell my mom that it's perfectly fine to schedule our family christmas when we don't have the SKs? Bc secretly I am in full support of that?!

It is not wrong, especially when the hosts prefer not to have savages at their event. I don't blame your mom, one bit.

"well their mom allows that behavior, so they don't know better..."

Absolute crap. Kids, at a very young age, figure out how to act, where, and who will put up with what... and who won't. If your fiance wants them included, he best teach them some manners. HE has taught them that they can act feral, anywhere, and get away with it.

If I make plans around not having SKs bc they are not scheduled to be with us, then find out last minute that they will be with us

Stop allowing this. No last minute bull.

Relationships involving people who have kids can be hard to navigate, especially for females. We are hardwired to be relational and seek inclusion, and since there's no template for these things, we often end up doing too much or putting up with too much.

In steplife you have to be able to speak up, stand up for yourself, and know what your role is and isn't. The way it's supposed to work is he's responsible for his kids, you're responsible for yours. Problems arise when we don't handle our responsibilities, have strong boundaries, or stay in our own lane. You need to draw some hard boundaries and employ new tactics if you want things to change. And quit being so darn nice! Nice gets you walked on in steplife.

It sounds as if your SO is a cr@ppy, ineffectual parent. He's not listening to you, and you're overfunctioning for him. It's time to make him feel the consequences of his inaction. Stop covering for him, and stop doing for his kids. Not enough dinner to go round because he can't plan ahead or communicate? Tell him there's stuff for sandwiches in the fridge, and leave him to it while you eat with your kids. If there's an event you want to take the family to but he can't get his act together, buy tickets for you and your bios and GO. Stop doing his parenting for him. Tell him that the skids aren't welcome at your family's for Christmas because their poor behavior ruins things for everyone else. Stop bailing him out. You should not be shopping for presents or cleaning up after them; that's his job, and he needs to be left to fully experience his kids in all their awful glory. Get out of the way, and let him suffer. 

After he's had a good amount time to get sick of caring for his monsters, have a come to Jesus with him. Sit him down, look him in the eye, and tell him you expect him to parent and control his children. Be sure to emphasize that you care about the skids and want them to have the social and life skills necessary to be happy and successful, but expect him to up his parenting game. Maybe throw in something about the kids need him to guide and teach them because BM doesn't? Whatever works. Just tell him how it has to be, and how it will be if things don't change. This may include taking separate cars on outings, laying the truth on him that you won't be going to restaurants with them because it's embarrassing, etc. Stop putting up with the nonsense, step back from the skids, and let your SO be inconvenienced, stressed, and embarrassed.

EDIT: and stop listening to the enabling, coddling inlaws. They have their own agenda, and don't have to live with the day to day nonsense you do.

Seems you cant win with the gifts so why waste your time. I used to buy my sd gifts in the beginning she would just throw them aside act ungrateful so I stopped buying gifts now I just leave it up too DH to buy her things. 

If the sks are annoying and your family just want to enjoy the day without all the drama so be it. You dont need to include them in your own family affairs if you dont want too. 

If people want to rock up on the door at the last minute that there problem.

That the thing with sk they think they can just come and go and change there mind. Everyone suppose to just drop everything and cater to there needs. Open there arms and make a fuss. 

You get nothing but S*&t in return. 

Sometimes you just got to say F**k it I'm going to do what I want to do. If these people cant get there stuff together tuff. 

Either way your always gonna be the evil step mother what ever you do. So you might as well do as you want. 

Perhaps skids don't need to go to your mothers house or get gifts from you.  They have a father who can shop for them, and they have bio grandparents to visit. It's perfectly acceptable to have parallel holidays and lives when skids are involved, especially if they are ill behaved.  We all want to be the Brady bunch, but in real life that arrangement is rare.  

DH asked me to help with xmas shopping one year. Adult skids snubbed the xmas gifts I got for gskids...and that was the last year I bought them anything. It's all on DH now, as it should have been all along.  Don't be the doormat, no one will appreciate your sacrifice.  

Until you stop catering to unreasonable demands they will continue to expect you to bend over backwards for everyone. This is what you have taught them will happen. 

The mistake was made the first time you accepted this behavior. It may be difficult for everyone to adapt now but they will if you are consistent and do not bow to pressure. 

You do not need to parent for your husband. He is capable of doing it himself. If he does not step up to the standard that you would expect that is on him to parent in his own way, don’t try to step in. 

I would not buy gifts for skids who whine and complain about them.  I'd give DH a head's up - "hey, your kids never like what I buy them, so I'm done with that.  If you want them to have presents you'll have to take care of it yourself."

I would not impose ill-behaved skids on my family.  Let DH know that holiday visits to your family will only be scheduled during skid-less times, and tell him why.  Let him know that if he can get his children under control, they might be welcome in the future.  If skids show up at a time when you're headed out to visit your family, go without DH and skids.

Tell DH that you will no longer be making special dishes for the skids, because you cannot rely on them being there to enjoy them.  Let him know that you'll go back to doing that once he starts scheduling and having them on a reliable schedule.  If you're not expecting them and they show up, don't change your plans or make anything special.  DH can entertain his kids and make them food if there's otherwise not enough.  If you buy tickets to something for skids and they don't show up, make DH reimburse you, and let him know that until they are on a reliable schedule you will no longer be including them in special outings.

It is ridiculous for you to have to live with uncertainty because he cannot work out a schedule.  Sure, things come up and schedules change occasionally, but it shouldn't be a constant thing.  Maybe if HE has to deal with the consequences of the lack of schedule, instead of it being your problem and driving you nuts, it will become a bigger priority for him.

No, none of your thoughts are wrong.  However, you are the mother of your own misery.  Your SKids are toxic, illbehaved, unmannered   crotch nuggets. Quit catering to them and give them and your DH clarity that they will not be included in any event in public or at someone else's home. Due to their ungrateful bratty crap they will also not receive any gifts other than gift cards to carefully selected stores that they will not find anything that interests them in. (E.g. The Wild Bird Store.)

Your DH needs clarity that until he finds his balls, mans up and starts dealing with this toxic crotch nuggets that they will not be part of any activity or celebration that you  are not confident that they will behave for. This means that from the moment they enter your presence that DH should be on them like stink on the shitty little crotch turds that they are and keep them firmly corralled. He needs to know that if he doesn't deal with them immediately and effectively that you will. That should scare DH and his spawn so much that they won't pull their historic crap and if they do he will fix it in a way that they would never again risk experiencing.

Since he will not do a schedule, his kids will not be included in anything. If they show up unscheduled, he can stay home with them and have you plaster his unused ticket to his forehead with duct tape.  If they don't show up as scheduled he should make sure to wear a helmet to avoid having you staple the tickets purchased for his absent toxic spawn to his forehead.

Quit facilitating this crap and put your foot up DH's ass.

Please.