So for background i honestly don't believe I'm a bad step mother at all. I have always gone out of my way to be kind, welcoming, friendly and have helped out a lot with DSCs over the years always doing what I can. Both DH and their Mum has asked me for help with things before and I've always been happy to. I have a really good relationship with the kids too, they message me and even call me sometimes when they aren't with us too just to chat or to ask for something like kids do 
Anyway, just wanted to mention the above so people know I'm really not some horrible push the kids out type.
We bought a house recently as we'd been renting for a while and wanted to get on the ladder.
When we first moved we agreed that DSC would have the biggest bedroom so that our joint DS (who is 1) could have some storage in there for his things as he gets more as he grows but he'd physically sleep in with us until we were ready to move (we are only hoping to be here for about 2-3 years). Both rooms are doubles, one is slightly larger which DSC are in but we can manage okay in ours with the double bed, wardrobes and a cot/toddler bed for DS. We'll hopefully then be in a position financially to move onto a 3 bed in the nearish future.
Anyway, the set up was that there would be a space left clear in DSCs room so that we could get some storage in there for DS's things, toys, clothes, etc... DSC also have bunk beds (which they wanted), drawers for their clothes, a table, computer, a chair, side tables etc.. and a TV with games console.
DH has now started saying they can get more stuff like a sofa for example in the space we agreed to leave for a bit of storage for DS. DSCs haven't even brought this up he is just mentioning it.
We've argued tonight because I reminded him that we were supposed to be getting storage for DS's stuff especially as he gets older, starts getting more toys etc... And hes told me I'm pushing his kids out, taking over their room, not "letting them have a nice room"?! Our son will be fine with his stuff in our room until he's 3 (it's cramped as it is because we took smallest for precisely this reason!!).
I'm really upset (and pissed off) I am NOT a bad step Mum I know that in my heart, the fact he's saying I'm try to push out his kids because I've asked for a small space for storage for DS which we agreed before we moved and is why DSC are in the bigger room in the first place?!
As for not letting them have a nice room..?! They have so much stuff in there they absolutely do not go without at all, TV, drawers, table computers etc...
This absolutely isn't anything against DSC, they've known they'd have to have some of their brothers things in their room from the start and they've always been fine about it.
But DHs comments tonight have really upset me. It's like I can't make any comment about anything even remotely relating to the kids without him getting defensive like I'm terrible and want to push his children out. I also feel annoyed for my son who, whilst I appreciate is only young now, will have no space in his home for his things and will just have everything crammed (or even the attic because there is literally not enough room for much more stuff in ours) into our room for the next 2 years.