S.S.
I was in that situation as a teenager, but I wanted to share my room with my baby brother.
I recommend explaining to your son that he needs to share the room until a time when you can either finish the basement or get a bigger place. I would think your 2 year old would be better suited in a room with a big brother and not a sister or parent. Sometimes siblings have to make sacrifices.
Your oldest son could also finish the basement for himself if he prefers to have his own space. I do not know which son of yours is autistic, but if it is not your older son, then he should be able to do some work down there himself.
Hope that helps & good luck!
Have a GREAT day!
S.