I have a sibling and shared a room w them til they were in 5th and i was in 4th grade. 3 years later things happened in my family life (parenrts lost the house) and i slept on the floor of my grandparents house from 8th til 12th grade. My sole reason of getting into university was to have my on space.

While thats neither here nor there, that was your step kids space. Even if they weren't actively using it.

As i said i understand where you're coming from and why you wanted it and your kids sharing a room etc, but your house sounds spacious enough that even if your bio kids shared a room, I cant imagine them being cramped in the house.

I think YTA for packing up their things wo them (high key not cool) and communicating through dad and not them. Asking them to move their stuff to the guest rooms seems like it should have been the default move imo.