Hi and welcome to the site.  

Photos - we do have a few small ones up of my adult bios and the SDs.  A few small ones - OK - a massive poster - no I would not agree to that. 

Parties - my DH used to occasionally join in with whatever BM was putting on for them, I never did.  He did the occasional pleasure trip for them, like to Alton Towers, again, I never went. 

We certainly did not pay any extra CS, DH vastly overpayed in any case so no.  

Snacks - if you don't want them helping themselves then put ALL the snacks in a place they can't get at, and hand them out as YOU see fit.  

Holidays - I never went on a holiday with the SDs - it would have been no holiday for me, just purgatory.  My DH occasionally took them away for a few days in this country (UK) and once he took them to New York for a week.  Only once in the 11 years they were coming regularly EOW.  

Presents - I don't see that kids have any say in what grandparents buy.  My parents are both long dead so it's not an issue anyway, but really it's inappropriate for any child to complain about gifts given by grandparents.  They are lucky to get one, if you ask me!  

Your way of dealing with Xmas sounds fair to me.  

I treat my stepkids the way I want my partner to treat my bio son. They are all close in age, so I treat them as much as possible the same way, though obviously they are all different people. 

There's no way I'd have a massive poster in the bedroom of any of the kids... Right now we have two separate houses but we are in the process of moving into one. I have some pics of my son in mine he has some in his. When we move in together there will be an equal amount of pics of kids up. I am taking new pictures as we make new memories and will be putting up a few of the three of them together. 

DH and I have a policy. No kid photos--bio or skid-- in our master bedroom. That is our private space. We have pics of the kids around the rest of the house, but that one room is off limits. Who wants to see a giant picture of any kids when they are trying to get it on??  Maybe he has his own space somewhere in the house where you can hang the photo.  I think he should be able be able to hang up a photo of his kids if he wants, but you two should compromise on the location in the house.

"What do you about their birthdays?" 

We celebrated the Skid's birthday as he was with us for every birthday from 2yo untill 17.  The SpermClan never celebrated his birthday.  Nor did they celebrate Christmas.  Something to do with their fringe Christian cult. SS was with his SpermClan for his 18th birthday.  We celebrated it once he came home from the final CO'd visitation. 

"Am I wrong that I can’t always find the money for him to go to every school play or assembly as it is 200 mike round trip!" 

No, you are not wrong.

"Would you give them their own bedroom, even though they aren’t even staying at the moment and if they do it’s 4 nights a month." 

If you have the space... sure. Why not?  If you don't have the space..... they get an inflatable matress in the den, etc.....  Full time resident kids get the space. Part timers... are accomodated as necessary.

"Do you pay any extra than csa? I personallly won’t but he gets guilt tripped."

We were the CP household and we made sure to never give the SpermClan access to any funds.  We learned early that any travel money we sent with SS when he was traveling to SpermLand for visitation would be stolen by the SpermClan. SS was expected to fill their gas tanks, pay for everyone to eat out, etc, etc, etc....

Eventually we stopped giving SS cash and just gave him a debit card that we would keep $Zero in unless he called and needed money.  Then we would immediately transfer money into that account.  We had fun with that one.  When the SpermClan saw what they thought was a credit card they went nuts and gave SS the bill for a big family lunch for 12 people.  There was no money in the account (by design) so they ended up having to keep 4 screaming kids and 8 older kids and adults waiting in the restaurant until SpermGrandPa could bring cash.  We had fun with that.  *diablo* 

"My partner wants to ask to have his eldest daughter for 2 weeks in the school holidays to get her swimming lessons as the mother doesn’t, and ‘because we pay for the 3 year old and 9 month old’ it’s a lot of money to fine the extra."

Why does he have to ask for time with his daughter?  Doesn't he have a visitation order?  Get the lessons on his time.  As for the money... I don't see anything unreasonable in DH spending money on his daughter when she is in  your home on visitation.  DH should not send additional money beyond CS to BM but the care and feeding of  his kid when his kid is with him is on him.  No different thatn the younger two. 

"I don’t like my steodaighters picking up their baby brother because he is a really heavy 9 month old plus they are always covered in nits."

This is two separate issues.  First is the lice.  They don't enter the home if they are infested.  They get "dipped" in the back yard (fully scrubbed with RID), Nit combed, their clothes get washed in pesticide detergent and dried at furnace temperatures, their belongings are thoroughly sprayed with Lice pesticide and only then do they enter the home.  The issue of picking up their brother... if they are of adequate size to do it... they should be allowed to IMHO.

"I have done them their own snack draw like I have for my eldest, but they always claim it’s never good enough compared to his. He has the yoyo bear fruit things, and the fruit shapes etc because he is still that age to have them and they are expensive so we get them like the Maryland cookie mini packs (as we do for my sons draw.) and sometimes Asda own bits which apparentlt it’s not branded so they won’t eat. So inkeeep catching them stealing stuff out of my boys draw! What would you do?"

Lock the drawer.... pretty simple.  Keep age appropriate snacks in each kid's drawer.  They eat their own stuff or they don't eat snacks. 

"What do you typically spend when you have your step kids?"

We fully supported SS-25 from 2yo-18yo.  He was in our home and had full access to the benefit of our marital resources.  Parents are responsible for supporting the children in their home.  My DW recieved $110/mo in CS for two years, then $133/mo in CS for 9 years, then $385/mo for 7 years.  Not enough to amount to more than a token of what it actually costs to raise a kid but... that money went into the family bucket just like my income and my bride's income.  So  you support the Skids when they are in your home. What the family does during that time... the Skids do too.  When the Skids are not in  your home, they are supported by the CS that  your DH pays.  No more, no less.

At least that is how we did it.

"Would you rake them on holidays abroad etc even if you knew they would ruin it for everyone, plus have to buy them both passports and expensive healthcare?"

If the kid was with us during scheduled trips... the kid went with us. If he was in SpermLand on visitation .... he didn't go.  Don't over complicate things. 

"Do you expect your parents to buy them Christmas and birthday stuff even after last time the eldest SD said ‘but my present is a lot smaller than hers and I have one less present (my parents bought them the same value presents.)"

We didn't expect my parents or any other extended family to do anything for the Skid... but they did. He is my parents eldest grandchild and my brother's eldest and only nephew and my nice and two nephews cousin.  As for the behavior... that needs to be confronted and dealt with through the application of age appropriate consequences.   

"Do you stop Christmas Day and not give the kids living with you a Christmas Day until the stepkids stay and have to together? Last year I did Christmas Day, and then we did a second Christmas Day a week alter and I kept some of the boys presents back from Christmas Day  to open too and my partner said that was wrong."

That is just stupid.  Christmas day is Christmas day.  You can certainly have a second one when the Skid's are with  you over the holiday season but to deny or postpone the celebration for resident children  just to include non resident children at a later date is wrong.  IMHO of course.  The non resident Skids no doubt have Christmas on Christmas day at their other home so why deny your own kids?  Holding an extra gift for resident children to open when non resident children are present is entirely reasonable.  So do it.

Just my thoughs of course.

Good luck.

Photos - Her family's pictures are all up and everywhere. She's managed to put pretty much all of my family photos away in the closet. She says I need to get some photos out, but I wonder why she does this. She totally dominates on this by moving things when I'm not home. 

Parties - I've bought her kids some nice stuff and shelled out hundreds for parties. They stopped appreciating what I was doing, so as of early this month, NO MORE. 

Child Support - My kids are adults who still live with their mom who lets them do so. Quite the enabler. I pay no CS any more.

Snacks - I've bought lots of groceries while DW is on business trips and SKids are supposed to be at their dad's - but these kids (to a little less degree) come over and eat the stuff I bought. I'm not too thrilled about that, as they tend to "cherry pick" and will rip a bag of potato chips all the way down the side and not close it back up - or they open a box and don't close it, so they get stale. It's a CONSTANT uphill battle.

Holidays - I've been on holidays with her and SKids to see her family. I buy them all gifts as I can.  

Presents - My folks give the SKids gifts. SKids are nice to them.

Thing is, I feel like her kids treat me semi-disrespectfully. I think in a lot of ways, DW treats me dismissively sometimes as well; she never really points out the good things I do for her kids. I work like a dog to do things around the house, and in return, they act very entitled. Example: I had earlier bought her son a REALLY NICE gift, and we told him that was going to be his Christmas and birthday gift. Christmas rolled around, and this kid had been whining incessantly about how slow MY Sony PS3 was (no apprecaition or acknowledgement that it was MINE, and he didn't treat it too well). She wanted to get him a new PS4, but waited... and they were all sold out. I found one and got it in. YAY. I paid for it, thinking she'd pay me back, but no... it was a gift "from all of us." Say what? Well... Forget that junk... I'm tired of feeling used.

Set boudaries early on. You know what feels right by how it feels. If you feel petty, put the shoe on the other foot and think if you'd feel different as the recipient of that. I get to speak my mind and sometimes do, but I feel like the three of them are taking advantage of me because I allowed it.

I have the same amount of photos of skid vs bio kid in our public area.  DH and I have separated bedrooms; I have my kids photos and awards in there. I'll give SD a gift card; DH spends the big money on her.  I spend the big money on our children's gifts and don't bother to ask DH for anything.

Photos - Its pretty equal, in our room though my husband has on picture on his nightstand of his son and I have an ultrasound photo of our daughter on mine. I wouldnt want a big poster of any child in my room. Thats my space away from the kids. However I do have a "baby's first year" frame of my daughter hanging up and nothing like that for my SS as his mother didnt take those kinds of pictures, but thats not my fault.

Birthdays - SS lives 5 hours away so we pick a weekend near his birthday and go over as a family to celecrate. My in laws live near my SS so it give them a chance to see all the kids as well, but my MIL does help with the cost of gas and we stay at her house, so we dont have to pay for food or a hotel. 

Bedroom - My SS has his own room, but we have the space and it is also our guest room. So he has toys and a dresser full of clothes in there, but if someone comes to visit we would set up an air mattress in his sisters room for him. 

CS- We pay what the state takes out of my husbands paycheck each month and no more. We will be paying for my daughter to go to preschool even though my SS isnt in preschool because we are only willing to pay half of that as we think his mother should pay the other half. 

Picking up the baby - My SS wasnt allowed to hold his sister (when she was younger) unless it was after he soaked in the tub for an hour and someone was sitting next to him. Itll be the same way when my son is born in a couple months. 

Snacks - I have them all locked up. Only my husband and I can get to them and we dictate who gets what and how much they get. There are certain snacks that are more expensive that I buy for my daughter because she is younger that I dont allow my SS to have because he goes through snacks like theyre water. 

Trips- I make sure to plan them around my SS visits because I dont like bringing him places. His behavior embarrasses me and so I would then not enjoy myself and neither would my daughter because of my frustration. 

Parents buying gifts - My family buys my SS presents and they spend an equal amount, they go based on dollar value not size or number of gifts. If my SS has a problem with it, thats to bad and he can than donate what he did get to sick children in the hospital that arent quite as lucky as he is to have so many people buying him gifts.

Christmas - I would NEVER postpone Christmas. It is my favorite holiday for one and it simply wouldnt be fair to my daughter. We do exactly what you did and have 2 Christmases and just keep some of my daughters presents for that day. We dont spend as much on my SS for Christmas though as he is getting 2 and our daughter only gets the one.