Yeah I think you're getting a rough ride, never. Different people have their own view on whether you're right to be annoyed depending on how they feel / would feel. And empathy can be touch and go on here, thick skin often required.
If I hadn't done quite a bit of research on step families, before marrying my 'childed' (?) hubby, I might have reacted similarly to you. And DSD used to forego jumping in my seat and simply force herself between us. And we all live together, so wasn't a weekend thing. She was 11 when we met and 16 now.
We've moved on from this issue and usually all get on quite well, but there are still struggles - we're going through a tough time right now. Tbh, DSD is often easier to get on with than DH whose major guilt / worry / withdrawal at the mo is impacting us all.
I also get the sacrifice element too when you're physically, emotionally, financially etc giving of yourself to your partner and child, often doing far more than the other parent. I'm not going to deny that there can be jealousy in step families. We're human. My DH has admitted that he feels left out of our (DSD and my) close chats. And I often wonder how DSD's mum feels (I go to the school plays, set up the work experience, go on the holidays etc...). I sometimes find it hard if DH is graceful to DSD but critical of me.
But...despite emphasizing with you on this one, I do think you might need to grieve and accept the sofa saga. As with loads of step issues, learn not to take it personally - you'll probably be better off for it. The kids didn't ask you to be in their dad's life, and as a stepchild myself, I can relate to that.
My friend, whenever she her hubby and his kids go out to a restaurant or anywhere requiring choosing seats, she pops to the loo first so when she comes back they've decided where to sit and she can settle herself wherever. I love that and have borrowed it. Actually this week I arrived first at a restaurant we booked and sat in the 'single' position (3 places set on a table for 4). But when DH and DSD arrived he moved the cutlery and sat next to me. So as I say we're beyond this now. If anything we're struggling to get DSD out of her bedroom to join us as a family.
Hope that helps in some way.x