Lol my post got deleted, not a surprise, there's someone who I believe always reports me where possible. I've asked MNHQ to explain how it broke talk guidelines as I don't know how it did.
In the meantime, I'm going to make my point in a different and hopefully non-deletable way.
You don't have to be in "poor SC mode" to see how a 7 year and 16 year old sharing a room could be problematic. It is very, very obvious. A 16 year old is nearly an adult, a 7 year old is "very much a young child. They are going to have different bedtimes and sleep and privacy requirements.
So the issue here is the age difference, not that one is a step-child.
It might not be practically feasible to give them their own rooms, but this arrangement is not working as neither the 7 year old nor the 16 year old's prime needs are met. The 7 year old needs to be able to sleep, and the 16 year old needs privacy and a place where he can game without disturbing the 7 year old.
I wouldn't like this either OP and I feel for your son and his step-brother. Definitely worth raising with your ex.