First off I have Feelings about this because I was the child in a turbulent custody battle during the 70s and 80s, so please bear with me!
It looks like your partner has grounds to request a review of the arrangement by the court. I'm thinking there are 2 grounds for this, firstly that the boy is no longer a baby and has different needs accordingly, and the second is that the mother remarried and moved away.
The needs of the child are paramount, so you'd need to show that he thrives with his dad and you. Does he eat and sleep well with his dad? Does he miss any school under the current arrangement? Does the travelling upset him? Are you expecting to marry after living together?
It sounds to me like you've handled things sensitively and haven't reneged on any of the stipulations of the current order and that the boy is happy, which are all in your favour. The sharing a room issue sounds like a threat, especially given that this is normal for him. Mum and new husband need to make sure he and his step sister don't share past the age of 10, but I'm guessing your 11 yo is a boy, so this won't be as much of an issue for you. She may be shooting herself in the foot with this one!
Just to finish by saying at age 6, I wasn't able to express my true feelings on how much time I spent with each of my parents because I was way too anxious about saying the wrong thing. I mainly wanted to stay with my half brother and have a normal family set up that didn't involve fighting and keeping secrets. Most of all, I wish that my parents could have put their differences aside for my sake and just been more grown up about the whole thing. They never, ever stopped bad mouthing each other. My mum still does it and my dad has been gone over 20 years! It affects me still. So yeah, if they can be amicable, that's the best thing possible for the son.
Best of luck OP xxx